Add background commands and desktop notifications

[nasser1941]

Closes #51.

What

• Background commands (src/lcode/jobs.py):
  • Starting one: bash(command, background=true) starts the process in its own process group. lcode keeps its output (capped at 1M characters) and returns a job id with the first output; a command that ends at once says so.
  • bash_output(id) returns what's new since the last read, with the job's status. It's allowed in plan mode.
  • bash_stop(id) stops the process group: SIGTERM, then SIGKILL. In the sandbox it stops the container process too.
  • /jobs lists them and /jobs stop <id> stops one. Subagents share the session's jobs. api.close_agent stops them all, which covers the terminal, -p, the Python API and lcode acp.
• Leftovers of &: a foreground command that starts something with & no longer hangs until the timeout while that process holds the output open. Once the command itself has ended (lcode's marker arrives), lcode waits half a second, stops what the command left running, and tells the model to use background=true. Commands like a & b & wait are unaffected.
• Notifications (src/lcode/notify.py): notify-send on Linux, osascript on macOS, and the terminal bell otherwise.
  • When: a request runs longer than notify_after seconds (30) and finishes, or lcode waits for an answer during such a request (a permission question or a plan).
  • Where: only in terminal sessions, not -p, the API or editors.
  • Settings: notify (on by default) and notify_after.
  • The notification hooks keep working, now through the same Agent.waiting / Agent.finished.
• Docs: Background commands and Notifications sections in the usage guide, plus the settings, the tools table and /jobs.

Tests

• tests/test_jobs.py (9 tests):
  • a job running until stopped, its children included;
  • the output cap;
  • the model starting, reading and stopping a job through the tools;
  • jobs stopping at session end and from /jobs;
  • leftovers of & being stopped, while a & b & wait is unaffected;
  • plan mode;
  • the notification commands and the bell fallback;
  • when long requests and waits notify, and when they don't;
  • plan approvals notifying.
• Real runs with Qwen3.6 35B through Ollama (-p, yolo):
  • "Start a web server … check it with curl … read its output … stop it": the first version of the tool description let the model use its own & and kill.
  • With the clearer description: bash(python3 -m http.server 8765, background=true), curl, bash_output, bash_stop. The answer quoted the server's access log, and the port was closed afterwards.
  • Notification: a terminal session with notify_after = 1. dbus-monitor showed the Notify call ("lcode is done", with the request and its 15 s).

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