docs: note optional local web launchagent

[shiny-code-bot]

Summary

• document that the optional local macOS LaunchAgent for mediaforce-web may be disabled at session start
• clarify that agents should check the local web server before using it and start it intentionally if needed
• note why macOS Login Items may show the item as uv

Validation

• git diff --check -- AGENTS.md
• pre-commit acceptance hook:
  • backend tests: 624 passed
  • CLI smoke: uv run mediaforce --help
  • frontend check/lint/unit tests/build
  • web route smoke