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.

1 files changed, 9 insertions, 0 deletions

diff --git a/zellij-utils/src/session_serialization.rs b/zellij-utils/src/session_serialization.rs
index 2b3f651c..75cd4ece 100644
--- a/zellij-utils/src/session_serialization.rs
+++ b/zellij-utils/src/session_serialization.rs
@@ -25,6 +25,8 @@ pub struct TabLayoutManifest {
     pub floating_panes: Vec<PaneLayoutManifest>,
     pub is_focused: bool,
     pub hide_floating_panes: bool,
+    /// Opaque agent-facing hint for what this TAB is for.
+    pub purpose: Option<String>,
 }
 
 #[derive(Default, Debug, Clone)]
@@ -87,6 +89,7 @@ pub fn serialize_session_layout(
 }
 
 fn serialize_tab(
+    tab_purpose: Option<String>,
     tab_name: String,
     is_focused: bool,
     hide_floating_panes: bool,
@@ -115,6 +118,11 @@ fn serialize_tab(
                     .entries_mut()
                     .push(KdlEntry::new_prop("focus", KdlValue::Bool(true)));
             }
+            if let Some(purpose) = tab_purpose {
+                serialized_tab
+                    .entries_mut()
+                    .push(KdlEntry::new_prop("purpose", purpose));
+            }
             if hide_floating_panes {
                 serialized_tab.entries_mut().push(KdlEntry::new_prop(
                     "hide_floating_panes",
@@ -652,6 +660,7 @@ fn serialize_multiple_tabs(
         let floating_panes = tab_layout_manifest.floating_panes;
         let hide_floating_panes = tab_layout_manifest.hide_floating_panes;
         let serialized = serialize_tab(
+            tab_layout_manifest.purpose.clone(),
             tab_name.clone(),
             tab_layout_manifest.is_focused,
             hide_floating_panes,