docs(chat): add AIRC migration inventory gates

[joelteply]

Summary

• add a generated chat-to-AIRC migration inventory for Replace Continuum chat/Postgres dependency with AIRC Rust SQLite substrate #1253 covering ORM hot paths, producers, consumers, AIRC interface gates, blockers, and performance evidence
• update the proof-gates doc to point at the generated inventory and align it with AIRC Rust transcript substrate work (Tailscale must be FIRST in install pipeline — grid connectivity before everything else #637/Factory job queue — RTOS-style task scheduling across grid nodes #638)
• add a focused doc contract spec so future edits keep the real hot paths and typed-adapter/no-table-coupling requirements visible

Verification

• npx tsx src/tests/unit/chat-to-airc-proof-gates-doc.spec.ts
• cd src && npx eslint tests/unit/chat-to-airc-proof-gates-doc.spec.ts --max-warnings 0 --quiet
• git diff --check
• precommit hook passed: TypeScript build, staged ESLint, browser ping
• pre-push hook passed: TypeScript clean, ESLint baseline held, Rust compile/tests with metal,accelerate

Notes

• pre-push reported native-arch Docker push failed but continued by hook policy; CI architecture verification should remain the merge gate for that.
• browser ping printed /bin/sh: tailscale: command not found, but the test passed.