feat: Phase 0 portability prep for open-standard support

Description

Phase 0 of the open-standard epic #66 — make the skills portable before publishing to skills.sh and before wiring tier-2 harnesses (Codex, Gemini CLI). Blocks the rest of the epic.

Workstreams

1. Namespacing + collision audit — flatten skill names to kebab-case under .agents/skills/; audit for cross-plugin collisions with vgv-ai-flutter-plugin; prefix vgv- where needed. Note the existing /review -> quality-review rename from PR #220.
2. Argument / path fallbacks — the $ARGUMENTS and ${CLAUDE_SKILL_DIR} symptom-described fallbacks partly landed via PR #220; finish applying them across all skills so bodies survive hosts that do not substitute.
3. MCP references — only context7; ensure skills that use it degrade gracefully where absent.
4. Reserved slash-command collision audit — check skill names against Codex, Gemini CLI, and OpenCode built-in commands (precedent: Copilot's built-in /review).
5. Agent-definition format conversion — all three tier-2 harnesses support subagents, so the review/plan/research panels port rather than degrade. Define a single source for agent definitions and convert per host: md -> TOML for Codex (~/.codex/agents/), md + YAML frontmatter for Gemini CLI (.gemini/agents), and markdown agents for OpenCode (mode: subagent) — the latter two are close to our current agents/*.md.
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, user-invocable, disable-model-invocation, effort, argument-hint, allowed-tools) are ignored rather than fatal. Add a CI lint that fails on malformed frontmatter so skills never vanish silently.

Requirements

• Skill names flat kebab-case, no cross-plugin collisions
• $ARGUMENTS / ${CLAUDE_SKILL_DIR} fallbacks applied across all skills
• context7 reference degrades gracefully where absent
• Reserved-command collision audit complete for Codex + Gemini CLI
• Agent-definition format conversion defined (md -> TOML Codex, md+frontmatter Gemini, md agents OpenCode)
• Every SKILL.md parses on Codex + Gemini + OpenCode; CI lint fails on malformed frontmatter

Additional Context

Part of epic #66. Blocks the skills.sh, Codex, and Gemini CLI items.