docs: add progressive disclosure documentation

[plutoless]

Summary

• Adds AGENTS.md (full contributor handbook + progressive disclosure load order) and CLAUDE.md (redirects to AGENTS.md) at repo root.
• Creates docs/ai/ tree: L0_repo_card.md, all 8 L1 files, 2 L2 deep dives (rtm_adapter_and_session.md, native_dependency_setup.md), RECIPE.md, and test-results.md.
• Documentation covers the React Native engine adapters, RTM subscribe ordering, Android emulator host alias, native linking, token security, and the keyless Python backend pipeline.

Test plan

• npx tsc --noEmit — 0 errors (executed)
• npm test --ci — 2 passed (executed)
• cd server && pytest -q (venv_test, agora-agents 2.3.0) — 2 passed (executed)
• 15/15 Q&A across 5 categories in docs/ai/test-results.md — all Pass
• Relative link check: 33 checked, 0 broken file targets

Note on branch contents

This branch contains two commits. The commit b8bc7f2 (fix: migrate recipe-client-rn-quickstart to agora-agents 2.3.x + construction smoke test) predates this docs work and was already on the branch before the docs commit. The docs commit (1fbbb0a) introduces only AGENTS.md, CLAUDE.md, and docs/ai/. If the server fix needs a separate review, it can be split out.