docs(ai): remove stale design documents

[kitlangton]

What

• Remove the obsolete AI design proposal, parity status, and call-site scratchpad.
• Refresh contributor guidance and remove the stale README link.
• Correct source comments for inclusive usage accounting, Usage.providerMetadata, protocol event decoding, cache placement/TTL behavior, shared Anthropic/Gemini routes, and Responses strict-schema support.
• Soften stale cache lookback/pricing claims, document OpenRouter cache markers, fix the tutorial route typo, and rename the stale registered-route test title.

Scope

Documentation, comments, and test naming only. This does not change executable code, types, test behavior, provider options, credentials, package entrypoints, or tutorial behavior from PR #43491.

Testing

• bunx prettier --check on all changed Markdown and TypeScript files
• bun run test test/tool-runtime.test.ts from packages/ai (25 passed)
• bun run lint <10 changed TypeScript files> (0 errors; 6 pre-existing warnings in cache-policy.ts)
• bun typecheck from packages/ai
• Push hook: full workspace bun turbo typecheck --concurrency=3 (33/33 tasks passed)
• git diff --check origin/v2...HEAD
• Verified the extracted TypeScript changes are comments/test naming only and no references to the deleted documents remain.