docs: rewrite README (all commands, subagents, skills) + add MIT LICENSE

[azmym]

Summary

Rewrites the README so it documents everything the /gemini-plugin: menu actually surfaces. The old README listed only the 5 slash commands and never documented the 5 subagents or the 9 skills, even though the slash menu shows all 14 command+skill entries.

README changes

• New "The /gemini-plugin: menu" section explaining that the namespace surfaces two kinds of entries: commands (fixed actions) and skills (capability guides loaded on demand).
• Slash commands table expanded with an Arguments column and the agent each command spawns; notes there is no gemini-review command (review runs via the gemini-consult skill to gemini-reviewer).
• New Subagents table: all 5 agents with model, role, and trigger, plus the fail-loud behavior.
• New Skills table: all 9 skills (2 routers + 7 capability), with the MCP tool each maps to.
• Refreshed the cost note to the current v0.4.x model tiers (added reviewer).
• Updated the nav links and the "Why use it?" list to mention diff/PR review.

Also

• Adds an MIT LICENSE file. plugin.json declared MIT and the README linked to LICENSE, but no LICENSE file existed (the link was broken). Copyright holder azmym, 2026.

All command, skill, agent names, and relative doc links verified to resolve.