Implement background command support for bash tool

- Added `background` option to `bash` tool to allow long-lived commands to run without blocking the turn.
- Introduced `bash_status` tool to check the status and output of background commands.
- Added `bash_stop` tool to explicitly stop running background commands.
- Updated command parsing to handle `/bash` commands for listing, stopping individual, and stopping all background commands.
- Enhanced session management to reap stale background commands on agent shutdown.
- Updated UI to reflect background command status and allow user interruption via ctrl-c.
- Added tests for background command functionality and command parsing.
- Documented new features and usage in background-bash.md.

src/session.ts

@@ -1004,7 +1004,11 @@ export class Session {
       // Files written this turn are now on disk; re-walk so the next prompt's
       // workspace list shows them without a restart.
       try { await this.refreshWorkspaceFiles(); } catch {}
-      onBashOutput(undefined);
+      // Keep the listener alive while a background process is running so the UI
+      // panel shows its output between turns. statusBackground always has the
+      // data either way (bg.tail); this just keeps the live panel up.
+      const { backgroundHandles } = await import('./tools');
+      if (backgroundHandles().length === 0) onBashOutput(undefined);
       await (this.pluginHost ?? this.opts.plugins)?.afterTurn();
       if (!this.opts.disableAutoLearn && this.messages.length >= 6) {
         this.learnTurns += 1;