channels: add open/close RPCs for dynamic pane management

Add open/close to the channel capnp schema. The tmux daemon implements
open by finding a pane by name (pane title or window name) and
attaching pipe-pane; close detaches and removes from state.

Tool handlers channel_open and channel_close added to the tool
registry.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

channels/tmux/src/main.rs

@@ -218,6 +218,98 @@ impl channel_server::Server for ChannelServerImpl {
     ) -> Promise<(), capnp::Error> {
         Promise::ok(())
     }
+
+    fn open(
+        &mut self,
+        params: channel_server::OpenParams,
+        _results: channel_server::OpenResults,
+    ) -> Promise<(), capnp::Error> {
+        let params = pry!(params.get());
+        let label = pry!(pry!(params.get_label()).to_str()).to_string();
+
+        // Check if already open
+        {
+            let s = self.state.borrow();
+            if s.pane_labels.contains(&label) {
+                return Promise::ok(());
+            }
+        }
+
+        // Find the tmux pane by name (window or pane title)
+        let pane_id = match find_pane_by_name(&label) {
+            Some(id) => id,
+            None => return Promise::err(capnp::Error::failed(
+                format!("no tmux pane named '{}'", label))),
+        };
+
+        info!("opening channel tmux.{} (pane {})", label, pane_id);
+
+        // Register in state
+        {
+            let mut s = self.state.borrow_mut();
+            s.pane_labels.push(label.clone());
+        }
+
+        // Start pipe-pane reader
+        let pane = PaneConfig { pane_id, label };
+        let reader_state = self.state.clone();
+        tokio::task::spawn_local(async move {
+            pipe_pane_reader(reader_state, pane).await;
+        });
+
+        Promise::ok(())
+    }
+
+    fn close(
+        &mut self,
+        params: channel_server::CloseParams,
+        _results: channel_server::CloseResults,
+    ) -> Promise<(), capnp::Error> {
+        let params = pry!(params.get());
+        let channel = pry!(pry!(params.get_channel()).to_str()).to_string();
+        let label = channel.strip_prefix("tmux.").unwrap_or(&channel).to_string();
+
+        let mut s = self.state.borrow_mut();
+        if let Some(pos) = s.pane_labels.iter().position(|l| *l == label) {
+            info!("closing channel tmux.{}", label);
+            s.pane_labels.remove(pos);
+            s.channel_logs.remove(&format!("tmux.{}", label));
+
+            // Disconnect pipe-pane — find the pane ID
+            if let Some(pane_id) = find_pane_by_name(&label) {
+                let _ = std::process::Command::new("tmux")
+                    .args(["pipe-pane", "-t", &pane_id])
+                    .output();
+            }
+        }
+
+        Promise::ok(())
+    }
+}
+
+// ── Pane lookup ──────────────────────────────────────────────
+
+/// Find a tmux pane by its title/name. Returns the pane ID (e.g. "%5")
+/// if found. Searches pane titles first, then window names.
+fn find_pane_by_name(name: &str) -> Option<String> {
+    let output = std::process::Command::new("tmux")
+        .args(["list-panes", "-a", "-F", "#{pane_id}\t#{pane_title}\t#{window_name}"])
+        .output()
+        .ok()?;
+    if !output.status.success() { return None; }
+
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    for line in stdout.lines() {
+        let parts: Vec<&str> = line.splitn(3, '\t').collect();
+        if parts.len() < 3 { continue; }
+        let pane_id = parts[0];
+        let pane_title = parts[1];
+        let window_name = parts[2];
+        if pane_title == name || window_name == name {
+            return Some(pane_id.to_string());
+        }
+    }
+    None
 }
 
 // ── Cleanup ───────────────────────────────────────────────────