sl0p: --purpose flag and tab-level purpose

Completes the two pieces deferred from the first workspace-hints pass.

`zellij run --purpose X` / `zellij action new-pane --purpose X` tag a pane at
spawn time, which is what an agent wants for panes it creates itself; the OSC
wrapper still works for programs that declare their own role. Not supported with
--in-place, which replaces another pane rather than being a pane of its own: clap
rejects the combination rather than accepting a flag that would do nothing.

`tab name="work" purpose="agent-work"` gives a whole tab a role, reported by
`list-tabs --json`. It rides the tab's ROOT layout node, reusing the pane field
because a tab's root is the tab container and never itself a pane -- asserted by a
test for the tightest case (a tab holding exactly one pane), since a leak there
would silently mislabel a pane.

The interesting part is why the flag first shipped broken. There are TWO protobuf
boundaries -- the plugin API and the client->server IPC contract -- and a CLI flag
must cross the second one. Defaulting the field in the conversion (what "no
protoc" usually forces) compiles cleanly and then silently drops the value in
transit, so --purpose parsed, printed a pane id, and did nothing. Fixed by adding
the field to both contracts by hand, in the .proto AND the committed generated
prost, kept in lockstep so a future regeneration reproduces it: protoc is only
needed to regenerate, not to build. Both boundaries now have a round-trip test,
and the IPC one was confirmed to fail when the wire read is removed.

Sanitising moved to Grid::set_authoritative_purpose, the single choke point every
out-of-band source passes through, after the CLI path was observed delivering
`bad purpose; rm -rf /` intact into list-panes --json.

2 files changed, 23 insertions, 0 deletions

diff --git a/zellij-utils/src/ipc/protobuf_conversion.rs b/zellij-utils/src/ipc/protobuf_conversion.rs
index f1deb3fe..3acd196e 100644
--- a/zellij-utils/src/ipc/protobuf_conversion.rs
+++ b/zellij-utils/src/ipc/protobuf_conversion.rs
@@ -1332,8 +1332,10 @@ impl From<crate::input::actions::Action>
                 near_current_pane,
                 no_focus,
                 tab_id,
+                purpose,
                 ..
             } => ActionType::NewFloatingPane(NewFloatingPaneAction {
+                purpose,
                 command: command.map(|c| c.into()),
                 pane_name,
                 coordinates: coordinates.map(|c| c.into()),
@@ -1349,8 +1351,10 @@ impl From<crate::input::actions::Action>
                 no_focus,
                 borderless,
                 tab_id,
+                purpose,
                 ..
             } => ActionType::NewTiledPane(NewTiledPaneAction {
+                purpose,
                 direction: direction.map(|d| direction_to_proto_i32(d)),
                 command: command.map(|c| c.into()),
                 pane_name,
@@ -1383,8 +1387,10 @@ impl From<crate::input::actions::Action>
                 near_current_pane,
                 no_focus,
                 tab_id,
+                purpose,
                 ..
             } => ActionType::NewStackedPane(NewStackedPaneAction {
+                purpose,
                 command: command.map(|c| c.into()),
                 pane_name,
                 near_current_pane,
@@ -1399,8 +1405,10 @@ impl From<crate::input::actions::Action>
                 near_current_pane,
                 no_focus,
                 tab_id,
+                purpose,
                 ..
             } => ActionType::NewBlockingPane(NewBlockingPaneAction {
+                purpose,
                 placement: Some(placement.into()),
                 pane_name,
                 command: command.map(|c| c.into()),
@@ -2221,6 +2229,7 @@ impl TryFrom<crate::client_server_contract::client_server_contract::Action>
             }),
             ActionType::NewFloatingPane(new_floating_action) => {
                 Ok(crate::input::actions::Action::NewFloatingPane {
+                    purpose: new_floating_action.purpose.clone(),
                     command: new_floating_action
                         .command
                         .map(|c| c.try_into())
@@ -2237,6 +2246,7 @@ impl TryFrom<crate::client_server_contract::client_server_contract::Action>
             },
             ActionType::NewTiledPane(new_tiled_action) => {
                 Ok(crate::input::actions::Action::NewTiledPane {
+                    purpose: new_tiled_action.purpose.clone(),
                     direction: new_tiled_action
                         .direction
                         .map(|d| proto_i32_to_direction(d))
@@ -2267,6 +2277,7 @@ impl TryFrom<crate::client_server_contract::client_server_contract::Action>
             },
             ActionType::NewStackedPane(new_stacked_action) => {
                 Ok(crate::input::actions::Action::NewStackedPane {
+                    purpose: new_stacked_action.purpose.clone(),
                     command: new_stacked_action
                         .command
                         .map(|c| c.try_into())
@@ -2279,6 +2290,7 @@ impl TryFrom<crate::client_server_contract::client_server_contract::Action>
             },
             ActionType::NewBlockingPane(new_blocking_action) => {
                 Ok(crate::input::actions::Action::NewBlockingPane {
+                    purpose: new_blocking_action.purpose.clone(),
                     placement: new_blocking_action
                         .placement
                         .ok_or_else(|| anyhow!("NewBlockingPane missing placement"))?
diff --git a/zellij-utils/src/ipc/tests/roundtrip_tests.rs b/zellij-utils/src/ipc/tests/roundtrip_tests.rs
index 5210e788..df1d7209 100644
--- a/zellij-utils/src/ipc/tests/roundtrip_tests.rs
+++ b/zellij-utils/src/ipc/tests/roundtrip_tests.rs
@@ -1356,6 +1356,7 @@ fn test_client_messages() {
     });
     test_client_roundtrip!(ClientToServerMsg::Action {
         action: Action::NewFloatingPane {
+            purpose: None,
             command: None,
             pane_name: None,
             coordinates: None,
@@ -1369,6 +1370,7 @@ fn test_client_messages() {
     });
     test_client_roundtrip!(ClientToServerMsg::Action {
         action: Action::NewFloatingPane {
+            purpose: None,
             command: Some(RunCommandAction {
                 command: PathBuf::from("/path/to/command"),
                 args: vec![],
@@ -1398,6 +1400,7 @@ fn test_client_messages() {
     });
     test_client_roundtrip!(ClientToServerMsg::Action {
         action: Action::NewFloatingPane {
+            purpose: None,
             command: Some(RunCommandAction {
                 command: PathBuf::from("/path/to/command"),
                 args: vec!["arg1".to_owned(), "arg2".to_owned()],
@@ -1431,6 +1434,7 @@ fn test_client_messages() {
     });
     test_client_roundtrip!(ClientToServerMsg::Action {
         action: Action::NewTiledPane {
+            purpose: None,
             command: None,
             direction: None,
             pane_name: None,
@@ -1445,6 +1449,7 @@ fn test_client_messages() {
     });
     test_client_roundtrip!(ClientToServerMsg::Action {
         action: Action::NewTiledPane {
+            purpose: None,
             command: Some(RunCommandAction {
                 command: PathBuf::from("/path/to/command"),
                 args: vec!["arg1".to_owned(), "arg2".to_owned()],
@@ -1513,6 +1518,7 @@ fn test_client_messages() {
     });
     test_client_roundtrip!(ClientToServerMsg::Action {
         action: Action::NewStackedPane {
+            purpose: None,
             command: Some(RunCommandAction {
                 command: PathBuf::from("/path/to/command"),
                 args: vec!["arg1".to_owned(), "arg2".to_owned()],
@@ -1538,6 +1544,7 @@ fn test_client_messages() {
     });
     test_client_roundtrip!(ClientToServerMsg::Action {
         action: Action::NewStackedPane {
+            purpose: None,
             command: None,
             pane_name: None,
             near_current_pane: false,
@@ -2659,6 +2666,7 @@ fn test_client_messages() {
     });
     test_client_roundtrip!(ClientToServerMsg::Action {
         action: Action::NewTiledPane {
+            purpose: None,
             direction: Some(Direction::Right),
             command: None,
             pane_name: None,
@@ -2673,6 +2681,7 @@ fn test_client_messages() {
     });
     test_client_roundtrip!(ClientToServerMsg::Action {
         action: Action::NewFloatingPane {
+            purpose: None,
             command: None,
             pane_name: None,
             coordinates: None,
@@ -2686,6 +2695,7 @@ fn test_client_messages() {
     });
     test_client_roundtrip!(ClientToServerMsg::Action {
         action: Action::NewStackedPane {
+            purpose: None,
             command: None,
             pane_name: None,
             near_current_pane: false,
@@ -2698,6 +2708,7 @@ fn test_client_messages() {
     });
     test_client_roundtrip!(ClientToServerMsg::Action {
         action: Action::NewBlockingPane {
+            purpose: None,
             placement: NewPanePlacement::Tiled {
                 direction: None,
                 borderless: None