docs(tools[pane/wait-for-channel]): restore wait-for safety contract intro

Hoist the composition pattern + status=$? idiom the mechanical split
dropped. Original prose lives at 970b000:docs/tools/waits.md:1-21.
The function docstring carries the idiom at the tool level, but the
page-level composition pattern (send_keys → wait_for_channel with
on-crash signal emission) was absent from the per-tool page.

Content restored:
- tmux wait-for channels as cheaper-than-polling synchronization.
- Composition pattern: send_keys a command that signals on exit,
  then wait_for_channel.
- The load-bearing safety idiom: ; status=$?; tmux wait-for -S NAME;
  exit $status, explaining edge-triggered wait-for would deadlock
  until timeout if the shell crashes before emitting.

Also strips a trailing '---\n## Signal' artifact left by the
mechanical splitter when it hit the old area page's next section
heading; signal_channel has its own page at pane/signal-channel.md.

Author: tony

docs/tools/pane/wait-for-channel.md

@@ -1,5 +1,19 @@
 # Wait for channel
 
+tmux's `wait-for` command exposes named, server-global channels that clients can signal and block on. These give agents an explicit synchronization primitive — strictly cheaper in agent turns than polling pane content via {tooliconl}`capture-pane` or {tooliconl}`wait-for-text`.
+
+The composition pattern: {tooliconl}`send-keys` a command that emits the signal on its exit, then `wait_for_channel`. The signal MUST fire on both success and failure paths or the wait will block until the timeout.
+
+```python
+send_keys(
+    pane_id="%1",
+    keys="pytest; status=$?; tmux wait-for -S tests_done; exit $status",
+)
+wait_for_channel("tests_done", timeout=60)
+```
+
+The `; status=$?; tmux wait-for -S NAME; exit $status` idiom is the load-bearing safety contract — `wait-for` is edge-triggered, so a crash before the signal would deadlock until the wait's `timeout`.
+
 ```{fastmcp-tool} wait_for_tools.wait_for_channel
 ```
 
@@ -18,7 +32,3 @@ rather than blocking indefinitely.
 
 ```{fastmcp-tool-input} wait_for_tools.wait_for_channel
 ```
-
----
-
-## Signal