feat: Phase 0 portability prep for open-standard support

Description

Phase 0 of the open-standard epic #51 — make the skills portable before publishing (#50) and before wiring tier-2 harnesses (#46 Codex, #125 Gemini CLI). Blocks the rest of the epic.

Workstreams

1. Guard MCP references — skill bodies reference mcp__dart__* and very-good-cli tools. Ensure they degrade gracefully (clear fallback instructions) where MCP is absent, so skills stay useful on harnesses without those servers.
2. Namespacing + collision audit — flatten skill names to kebab-case under .agents/skills/; audit for cross-plugin collisions with vgv-wingspan; prefix vgv- where needed.
3. Argument / path fallbacks — add symptom-described fallbacks for $ARGUMENTS and ${CLAUDE_SKILL_DIR} so bodies survive hosts that do not substitute them (describe the symptom, never name the token, or Claude Code substitutes it inside the explanation).
4. Hook adaptation groundwork — factor the enforcement hook scripts so they can be re-pointed to Codex (hooks.json / [hooks]) and Gemini CLI (Claude-hook migration) with minimal per-host divergence. OpenCode uses a JS/TS plugin API rather than the Claude shell contract, so its hook is a separate rewrite — keep the enforcement logic isolated enough to reimplement.
5. Reserved slash-command collision audit — check skill names against Codex, Gemini CLI, and OpenCode built-in commands (precedent: Copilot's built-in /review).
6. SKILL.md frontmatter compatibility — Gemini CLI silently skips a SKILL.md if the frontmatter is not the first thing in the file, if name/description is missing, or if the --- delimiters are off. Validate every skill parses on Codex + Gemini + OpenCode, and confirm our extra fields (when_to_use, allowed-tools, argument-hint, etc.) are ignored rather than fatal. Add a CI lint that fails on malformed frontmatter so skills never vanish silently.

Requirements

• MCP references degrade gracefully where MCP absent
• Skill names flat kebab-case, no cross-plugin collisions
• $ARGUMENTS / ${CLAUDE_SKILL_DIR} fallbacks in place
• Hook scripts factored for multi-host adaptation
• Reserved-command collision audit complete for Codex + Gemini CLI
• Every SKILL.md parses on Codex + Gemini; CI lint fails on malformed frontmatter
• All CI/CD checks are passing.
• There is no drop in the test coverage percentage.

Additional Context

Part of epic #51. Blocks #50, #46, #125.