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/helpers.ts

@@ -55,7 +55,12 @@ export function testHooks(over: Partial<AppHooks> = {}): AppHooks {
       remove: async (name) => `removed ${name}`,
     },
     initPrompt: 'write AGENTS.md',
-  scaffoldWorkflow: () => [],
+    scaffoldWorkflow: () => [],
+    backgroundCommands: {
+      list: () => 'no background commands',
+      stop: async (handle) => `stopped ${handle}`,
+      stopAll: async () => 'stopped all',
+    },
     history: [],
     recordPrompt: () => {},
     ...over,