feat: add gemini-reviewer agent + gemini-consult dispatch rule (v0.4.0)

[azmym]

Summary

Adds a 5th Gemini agent and a dispatch-rule skill, ported additively from the IPTV project's hand-tuned gemini-assistant setup. Existing hooks, the four existing agents, and brainstorm-on-by-default are untouched.

• gemini-reviewer (5th agent): generalist diff/PR review (security, threading correctness, library/version drift, doc accuracy, dead code, complexity), the one IPTV "code review" mode the plugin lacked. Sonnet, maxTurns 10, structured JSON verdict. Advisory: changes_requested surfaces inline but does not block, since it is dispatched manually rather than by a hook.
• gemini-consult (9th skill): the routing rule the main Claude reads, when to consult, which of the five agents to route to, a one-consult-per-turn cap on manual dispatches (hooks are a separate, uncounted channel), plus the ported disagreement protocol and "what you are NOT" guards.

Changes

• New: agents/gemini-reviewer.md, skills/gemini-consult/SKILL.md
• hooks/hooks.json: gemini-reviewer added to the SubagentStop matcher (advisory)
• Version 0.3.0 to 0.4.0; CHANGELOG entry
• rules/using-gemini.md, README, docs/index.md, docs/reference/{subagents,skills,architecture,hooks}.md, skills/gemini-when-to-use updated for the 5 agents / 9 skills counts
• tests/{agents,skills,hooks-triggers}.bats extended

Testing

bats tests/ : 76 tests, all green locally (CI runs Ubuntu + macOS).