feat(cli): background task observability and /bg namespace

[Dusk1e]

Summary

This PR improves CLI background task observability and fixes the /bg alias behavior. Background tasks launched with /background <prompt> were previously run in isolated sessions with no durable, user-visible state.

Fixes #8568

Why

Previously, the /background command was a "black box" with several limitations:

• No structured task logs or file change lists.
• No way to inspect task status from the foreground CLI.
• No foreground session context after completion (the agent didn't know what happened).
• /bg status was incorrectly parsed as a new background prompt.

Changes

1. Persisted Artifacts

Added profile-safe background task artifact storage under HERMES_HOME/background_tasks/<task_id>/:

• manifest.json: Task metadata and current status.
• log.jsonl: Structured event stream of activity.
• summary.txt & files.txt: Final result and changed file reporting.

2. New /bg Subcommand Namespace

Registered /bg as a dedicated subcommand area to prevent prompt fall-through:

• /bg status [task-id]
• /bg log <task-id>
• /bg files <task-id>
• /bg kill <task-id>

3. Context Awareness

• Session Injection: Lifecycle events (start/finish) are now injected as SYSTEM messages into the foreground session history.
• Workspace Diffs: Added file tracking via before/after workspace snapshots.
• UI Updates: Added active background task count to the /status command.

How To Test

1. Start the CLI and run: /background build a small HTML site
2. While running, verify status: /bg status
3. After completion, inspect results:
   • /bg log <task-id>
   • /bg files <task-id>
4. Ask the foreground agent: "What did the background task do?" (Confirm it has session context).
5. Verify files exist in ~/.hermes/background_tasks/<task-id>/.

Tests Run

Passed 38 targeted tests:

python -m pytest tests/cli/test_cli_background_commands.py \
  tests/cli/test_cli_background_tui_refresh.py \
  tests/cli/test_cli_status_command.py \
  tests/cli/test_cli_save_config_value.py \
  tests/gateway/test_background_command.py -q

[teknium1]

Closing in favor of the minimum-viable fix in PR #27175 (merged), which addresses the core UX gap that motivated #8568 with ~25 LOC: a live ▶ N indicator in the status bar showing running /background tasks.

Your implementation here was clean and complete — artifacts under background_tasks/<id>/, /bg status|log|files|kill subcommands, foreground SYSTEM-note injection, 38 passing tests. After reviewing it against the actual user complaint (people asking the agent about bg tasks because there's no visible signal one exists), we concluded a status-bar indicator solves the root cause without the larger feature footprint. The richer inspection surfaces can be added later if demand materializes.

Thank you for the contribution — closing this with credit.