fix(coding-agent): drain follow-ups queued during agent_end

When an extension queues a follow-up during `agent_end` it
gets stuck on the follow-up queue until after the next user
message.

Add a hasQueuedMessages() check to _handlePostAgentRun
so the existing while/continue loop drains them.

- One-line fix in _handlePostAgentRun
- Integration test in agent-session-queue
- git-merge-and-resolve example extension with tests

packages/coding-agent/docs/extensions.md

@@ -2553,6 +2553,7 @@ All examples in [examples/extensions/](../examples/extensions/).
 | `custom-compaction.ts` | Custom compaction summary | `on("session_before_compact")` |
 | `trigger-compact.ts` | Trigger compaction manually | `compact()` |
 | `git-checkpoint.ts` | Git stash on turns | `on("turn_start")`, `on("session_before_fork")`, `exec` |
+| `git-merge-and-resolve.ts` | Fetch, merge, and resolve conflicts | `on("agent_end")`, `exec`, `sendUserMessage` |
 | `auto-commit-on-exit.ts` | Commit on shutdown | `on("session_shutdown")`, `exec` |
 | **UI Components** |||
 | `status-line.ts` | Footer status indicator | `setStatus`, session events |