chore: harden release docs generation

[hyochan]

Summary

• reuse packages/kit/public/llms.txt as the canonical IAPKit section in the main llms-full reference
• run agent drift checks when the Kit source or generated LLM references change
• refresh generated references with the completed release-train versions
• migrate both Google release jobs to gradle/actions/setup-gradle v6

Verification

• agent compiler tests: 7 passed
• agent TypeScript typecheck
• docs TypeScript typecheck and production build
• docs Prettier check
• docs consistency audit
• non-Godot SDK parity audit
• stable release-state audit
• workflow YAML parse
• two clean review-self snapshots five minutes apart

[gemini-code-assist]

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