Skills 4/6: a Skills tab in /settings, and /skill on and off #1406

Step 4 of #1277. It lands in the single pull request for #1277 (branch feat/1277-skills).

What

A Skills tab in /settings, one row per discovered skill:

• a mark when the skill lives in codeaf's own folder
• the name
• the size, in bytes and estimated tokens
• its state: on here, on for this project, on globally, off

A skill with a warning shows it dimly. A shadowed skill shows what owns its name. A skill declaring allowed-tools shows the list. The tab does not say which harness's folder a skill came from.

Keys: Enter cycles the row's state for this conversation. A second key sets it for the project or globally.

Commands: /skill on <name> and /skill off <name> take --project or --global (default: this conversation). They go through the same engine door as the tab. /skills opens the tab.

Acceptance, end to end

Real binary in tmux with fixture skills:

• /skills opens the Skills tab listing every fixture skill with its size and state.
• Toggling a row globally writes ~/.codeaf/skills.json, and the next request's catalog includes the skill.
• /skill off pdf-processing --project writes <project>/.codeaf/skills.json, and the next request's catalog drops it.
• The tab and /skill never disagree about a state.
• The manual answers "how do I turn a skill on for just this project?".