feat: skill command group + bundled SKILL.md

[andyhtran]

Summary

• Ship cct as a Claude Code skill so agents auto-discover the tool without the user manually copying SKILL.md into ~/.claude/skills/cct/. Skill content is embedded in the binary; cct skill install creates a symlink at ~/.claude/skills/cct -> ~/.cache/cct/skills/cct, and the live copy auto-syncs from the embedded version on every cct invocation — brew upgrade cct keeps the on-disk skill aligned with the binary, no reinstall needed.
• Add cct skill install / uninstall / status / nudge subcommands. Install is idempotent and refuses to overwrite a foreign file or symlink. Uninstall removes only the symlink, preserves the live copy for fast reinstall. Status reports install/sync/nudge state with --json for tooling.
• Until installed, every cct invocation prints a one-line install hint to stderr (rate-limited to once per 24h, dismissible via cct skill nudge off). Skill content is reordered by real session-history usage frequency (search > export > info > list) with explicit anti-triggers for grep ~/.claude/projects/ and the non-existent cct show. Includes references/commands.md and references/search-syntax.md with full JSON schemas for stats, list, and search so agents stop guessing field names.

Test plan

• just ci passes (gofumpt + golangci-lint + go test)
• On a fresh ~/.claude/skills/ (no existing cct/ entry), cct skill install succeeds and reports Installed cct skill at ~/.claude/skills/cct
• cct skill status shows Installed: yes and Sync: in sync with embedded version
• ls -la ~/.claude/skills/ shows cct as a symlink (not a regular directory) pointing at ~/.cache/cct/skills/cct
• Frontmatter visible through the symlink: head -4 ~/.claude/skills/cct/SKILL.md shows the name: cct block
• cct skill install a second time is a no-op (idempotent — no error, exit 0)
• Pre-create a regular file at ~/.claude/skills/cct/SKILL.md and confirm cct skill install refuses with a clear error
• cct skill uninstall removes the symlink; ~/.cache/cct/skills/cct/ survives
• On a fresh state with no symlink, cct version prints the install nudge to stderr; running cct version again immediately is silent (24h rate limit)
• cct skill nudge off silences the nudge persistently; cct skill nudge on re-enables; cct skill nudge status reports current state
• cct skill status --json emits a stable, parseable schema with installed, embedded_hash, live_hash, in_sync, nudge_enabled

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