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, 14 insertions, 0 deletions

diff --git a/zellij-utils/src/kdl/kdl_layout_parser.rs b/zellij-utils/src/kdl/kdl_layout_parser.rs
index 9e823849..41b4a430 100644
--- a/zellij-utils/src/kdl/kdl_layout_parser.rs
+++ b/zellij-utils/src/kdl/kdl_layout_parser.rs
@@ -145,6 +145,7 @@ impl<'a> KdlLayoutParser<'a> {
             || property_name == "min_panes"
             || property_name == "exact_panes"
             || property_name == "hide_floating_panes"
+            || property_name == "purpose"
     }
     pub fn is_a_reserved_plugin_property(property_name: &str) -> bool {
         property_name == "location"
@@ -1261,10 +1262,16 @@ impl<'a> KdlLayoutParser<'a> {
             },
             None => vec![],
         };
+        // Tab-level purpose rides the tab's ROOT layout, which is the tab
+        // container and never itself a pane (asserted by
+        // sl0p_tab_root_layout_is_never_itself_a_pane).
+        let tab_purpose = kdl_get_string_property_or_child_value!(kdl_node, "purpose")
+            .and_then(|s| crate::input::layout::sanitize_pane_purpose(s));
         let mut pane_layout = TiledPaneLayout {
             children_split_direction,
             children,
             hide_floating_panes,
+            purpose: tab_purpose,
             ..Default::default()
         };
         if let Some(cwd_prefix) = &self.cwd_prefix(tab_cwd.as_ref())? {
diff --git a/zellij-utils/src/kdl/mod.rs b/zellij-utils/src/kdl/mod.rs
index 358fa3e2..98743d67 100644
--- a/zellij-utils/src/kdl/mod.rs
+++ b/zellij-utils/src/kdl/mod.rs
@@ -576,6 +576,7 @@ impl Action {
                     });
                 } else if string == "stacked" {
                     return Ok(Action::NewStackedPane {
+                        purpose: None,
                         command: None,
                         pane_name: None,
                         near_current_pane: false,
@@ -2067,6 +2068,7 @@ impl TryFrom<(&KdlNode, &Options)> for Action {
                     .and_then(|c_m| kdl_child_bool_value_for_entry(c_m, "borderless"));
                 if floating {
                     Ok(Action::NewFloatingPane {
+                        purpose: None,
                         command: Some(run_command_action),
                         pane_name: name,
                         coordinates: FloatingPaneCoordinates::new(
@@ -2088,6 +2090,7 @@ impl TryFrom<(&KdlNode, &Options)> for Action {
                     })
                 } else if stacked {
                     Ok(Action::NewStackedPane {
+                        purpose: None,
                         command: Some(run_command_action),
                         pane_name: name,
                         near_current_pane: false,
@@ -2096,6 +2099,7 @@ impl TryFrom<(&KdlNode, &Options)> for Action {
                     })
                 } else {
                     Ok(Action::NewTiledPane {
+                        purpose: None,
                         direction,
                         command: Some(run_command_action),
                         pane_name: name,
@@ -6521,6 +6525,7 @@ impl TabInfo {
             optional_int_node!("selectable_floating_panes_count", usize).unwrap_or(0);
         let tab_id = optional_int_node!("tab_id", usize).unwrap_or(0);
         Ok(TabInfo {
+            purpose: None,
             position,
             name,
             active,
@@ -6960,6 +6965,7 @@ fn serialize_and_deserialize_session_info_with_data() {
         name: "my session name".to_owned(),
         tabs: vec![
             TabInfo {
+                purpose: None,
                 position: 0,
                 name: "tab 1".to_owned(),
                 active: true,
@@ -6981,6 +6987,7 @@ fn serialize_and_deserialize_session_info_with_data() {
                 has_bell_notification: false,
             },
             TabInfo {
+                purpose: None,
                 position: 1,
                 name: "tab 2".to_owned(),
                 active: true,