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.

test/commands.test.ts

@@ -158,3 +158,22 @@ test('/registry add with no name returns usage rather than fetching anything', (
 test('a bare word after /registry is treated as a search', () => {
   expect(parseCommand('/registry migration')).toEqual({ type: 'registry', action: 'search', arg: 'migration' });
 });
+
+test('/bash with no verb lists background commands', () => {
+  expect(parseCommand('/bash')).toEqual({ type: 'bash', action: 'list' });
+  expect(parseCommand('/bash list')).toEqual({ type: 'bash', action: 'list' });
+});
+
+test('/bash stop carries the handle', () => {
+  expect(parseCommand('/bash stop 3')).toEqual({ type: 'bash', action: 'stop', arg: '3' });
+});
+
+test('/bash stop all stops everything', () => {
+  expect(parseCommand('/bash stop all')).toEqual({ type: 'bash', action: 'stop-all' });
+});
+
+test('/bash with an unknown verb returns usage', () => {
+  const out = parseCommand('/bash frobnicate');
+  expect(out.type).toBe('info');
+  expect((out as { text: string }).text).toContain('usage: /bash');
+});