feat(skill): integrate kitup for bundled agent skill install

[samzong]

What's changed?

• Add gmc skill install via kitup's go-cobra adapter
• Embed skills/gmc/SKILL.md into the binary with go:embed
• Add install coverage in cmd/skill_test.go
• Regenerate man pages for the new command surface

Why

• Users should be able to install the bundled gmc Agent Skill into local coding agents with the same safe, conflict-aware workflow kitup provides elsewhere

Verification

• make check

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Reject positional args for skill install

Because the kitup Cobra adapter used here constructs the install subcommand without an Args validator, Cobra accepts and ignores any positional arguments. For example, gmc skill install codex --yes treats codex as an unused extra arg rather than the target agent; with --yes it falls back to auto-detected agents and can write to different or multiple skill directories. Please wrap the returned command or set the install subcommand to cobra.NoArgs before registering it.

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