fix(skills): align /skills command handler with advertised subcommands

[wkramme]

🐶 What’s fixed

/skills autocomplete/help advertised subcommands (toggle, help, refresh) that the active slash-command handler did not actually support. Users would see errors like:

• Unknown subcommand: toggle

This PR aligns implementation with the advertised UX and keeps behavior consistent.

✅ Changes

• Added /skills toggle handling in agent_skills slash callback
  • Toggles global skills integration state
  • Emits clear enabled/disabled status output
• Added /skills refresh handling
  • Forces skill re-discovery and reports discovered/valid counts
• Added /skills help handling
  • Prints available subcommands and descriptions
• Updated unknown-subcommand usage text to include all supported options
• Updated callback tests to cover:
  • toggle path
  • refresh path
  • help path
  • usage text expectations

🧪 Test evidence

Ran targeted tests:

• uv run pytest -q tests/plugins/test_agent_skills_callbacks_coverage.py
• uv run pytest -q tests/command_line/test_skills_completion.py tests/test_hook_manager.py -k "skills or /skills or test_custom_command"
• uv run pytest -q tests/plugins/test_agent_skills_callbacks_coverage.py tests/command_line/test_skills_completion.py -k "skills"

All passing in local run.

Why this matters

This removes command-surface inconsistency and avoids user confusion where completion/help claims a command exists but execution rejects it.