feat(skill): add a kit skill command to install the kit agent skill

[youdie006]

Description

Coding agents (Claude Code, Codex, Cursor, ...) learn kit only from flag-oriented --help, which says nothing about workflows. This adds a kit skill command that installs an embedded SKILL.md — covering Kitfile authoring and the pack/tag/login/push/pull/unpack workflow, plus common errors — into the agent skills directories.

• New pkg/cmd/skill command, registered in cmd/root.go alongside the other subcommands.
• The SKILL.md is go:embed'd so it always matches the installed kit version and works air-gapped, exactly as the issue asks.
• It reuses the existing --as-skill installer (libskill.InstallSkill) and agent-directory resolution, so it lands in .claude/skills, the shared .agents/skills, etc., with the same --agents / -d / -o / -i flags as kit unpack --as-skill. No agent-dir logic is duplicated.
• Every section of the embedded SKILL.md is grounded in the real command implementations and Kitfile validation rules (no invented flags/behavior).
• docs/src/docs/cli/cli-reference.md regenerated via the repo's own docs/src/docs/cli/generate.sh.

Scoped to the core command per the small-PR principle. The issue's secondary ask — publishing the skill as a ModelKit — is left to a follow-up.

Tests (pkg/cmd/skill/cmd_test.go): installs the embedded skill to .claude/skills/kitops/SKILL.md; multi-agent install; unknown-agent error; a frontmatter drift guard. go build ./..., go vet, go test, and gofmt all pass; also smoke-tested with the built binary (writes the skill, overwrite / ignore-existing guards work, --help renders).

Design note: I named the skill/dir kitops; if you'd prefer something more action-oriented (e.g. using-kit) it's a one-line change to the skillName const + the frontmatter name (a test guards they stay equal).

Linked issues

Refs #1249

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[amisevsk]

Nicely done @youdie006, thank you!

I have one note on the cli-reference.md file, but apart from that it looks good to me. I'd like @gorkem's review on the SKILL.md, though I have no concerns personally and think it's a solid starting point.

[amisevsk]

A bit of a strange feature in this repository: this CLI reference doc is served at https://kitops.org/docs/cli/cli-reference/ based on the repository HEAD (i.e. not the last release). As a result, merging this update to the cli-reference.md file will update the public docs immediately, even though the skill command is not yet released.

Please revert this change in the PR; instead, this file will be automatically updated on the next Kit release through CI actions.

[amisevsk]

@gorkem I'd like your opinion on this SKILL.md for KitOps -- you're better at writing these than I am.