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>

schema/channel.capnp

@@ -56,4 +56,11 @@ interface ChannelServer {
 
     # List available channels and their status.
     list      @3 () -> (channels :List(ChannelInfo));
+
+    # Open a channel — start monitoring. Daemon-specific semantics:
+    # tmux: find pane by label name, attach pipe-pane.
+    open      @4 (label :Text) -> ();
+
+    # Close a channel — stop monitoring and clean up.
+    close     @5 (channel :Text) -> ();
 }