bash: Add run_in_background parameter

[evansenter]

Summary

Add run_in_background parameter to bash tool to run commands asynchronously.

Current Behavior

All bash commands block until completion or timeout.

Target API

{
  "command": "npm run build",
  "run_in_background": true
}

Returns immediately with a task ID that can be checked later (requires KillShell tool - see #67).

Use Case

• Long-running builds
• Watching file changes
• Running dev servers while doing other work

Implementation Notes

• Return task_id in response
• Store spawned process handle
• Need companion KillShell tool to manage background tasks