Skill: install/check/uninstall subcommands (#37)

[MisterGC]

Implements issue #37 (description rewritten on the issue to match the agreed design).

Three new sub-subcommands under grafli skill:

• grafli skill install [all|claude|codex|opencode] — copy the bundled SKILL.md into the user-level skill directory for each target. Bare invocation prompts per target. Positional installs only that target. all installs everywhere unattended.
• grafli skill check [all|claude|codex|opencode] — report install status per target. Bare invocation checks every target with no prompting. Human-readable by default; --json for machines.
• grafli skill uninstall [all|claude|codex|opencode] — remove the installed grafli/ skill directory per target, symmetric to install.

Behaviour highlights:

• Version detection via <!-- grafli skill version: X.Y.Z --> injected after the YAML frontmatter at install time; hash compare on canonical (version-line-stripped) content distinguishes five states: ok, stale, modified, unknown, missing.
• Flags: --force (skip prompts), --dry-run (preview without writing) on install/uninstall; --json on check.
• Non-TTY stdin without --force → refuses with a hint instead of guessing.
• Missing parent dir (target tool likely not installed) → prompts before creating, unless --force is passed.
• Targets:
  • claude → ~/.claude/skills/grafli/SKILL.md
  • codex → ~/.agents/skills/grafli/SKILL.md
  • opencode → ~/.config/opencode/skills/grafli/SKILL.md
• Backwards-compatible: bare grafli skill, grafli skill -o FILE, grafli skill --where still work as before.

Out of scope for this PR (deferred per design notes on the issue): --symlink install mode, project-level installs, additional targets.

Test plan

• grafli skill check reports all three as missing on a clean machine (exit 0)
• grafli skill install all --dry-run lists planned writes (exit 1 since drift exists)
• grafli skill install claude < /dev/null refuses with non-zero exit
• After grafli skill install all --force, check reports all three ok (exit 0)
• Edit one installed SKILL.md by hand → check reports modified
• grafli skill uninstall all --force cleanly removes the three directories
• 13 new unit tests cover stamping, canonicalisation, all 5 status states, write/remove primitives (full suite at 338)