Skills 5/6: /skill add and /skill remove, from a folder or a git URL #1407

Step 5 of #1277. It lands in the single pull request for #1277 (branch feat/1277-skills).

What

/skill add ~/some/folder                    copies it into ~/.codeaf/skills/<name>/
/skill add ~/some/folder --project          into <project>/.codeaf/skills/<name>/
/skill add https://github.com/o/r           clones, finds every SKILL.md, copies each
/skill add https://github.com/o/r/tree/main/skills/pdf   one folder out of a repo

• A URL is cloned to a temporary folder with the git on PATH, and the skills are copied out. Nothing in it is executed.
• A name already present asks before it is overwritten.
• /skill remove <name> removes only from codeaf's own folders, never from another harness's. It never touches a folder in ~/.codeaf/skills that has no SKILL.md: the resident keeps bin/ and its promoted commands there.

Acceptance, end to end

Real binary in tmux, no network:

• /skill add <fixture folder> puts ~/.codeaf/skills/<name>/SKILL.md on disk, and the skill appears in the picker and the Skills tab with the codeaf mark.
• /skill add file:///…/bare.git (a local bare repository holding two skills) copies both.
• A …/tree/<ref>/<path> URL copies only that folder.
• /skill remove <name> for a skill that lives only in ~/.claude/skills is refused with a message naming where it lives. ~/.codeaf/skills/bin/ survives every remove.