Strengthen agent rules from APIError porting learnings

[parthban-db]

🥞 Stacked PR

Use this link to review incremental changes.

• stack/update-agent-metadata-3 [Files changed]
  • stack/api-execute-retrier [Files changed]
    • stack/update-agent-metadata-4 [Files changed]

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What changes are proposed in this pull request?

Gaps found during the APIError porting PR (#13) where repeated back-and-forth was needed. These rule updates make the expectations explicit so future agents get it right on the first pass.

• porting.mdc — Add critical thinking section (don't blindly copy Go code that exists only because of Go language requirements like Unwrap()), factory functions vs constructors (use factory when Go returns nil), no Go references in code or comments.
• testing.mdc — Never silently skip assertions (use expect.fail() not return), test all outputs (don't skip "obvious" fields), use real types for expected values (not anonymous objects), preserve existing test structure (don't rename fields or restructure committed tests).
• typescript.mdc — @private JSDoc pattern for internal methods, API surface minimization (only export from index.ts what consumers need).
• CLAUDE.md — Stay in scope (don't change logic you weren't asked to change), match existing patterns (check committed code before inventing new conventions), strengthen the requirement for authoritative sources.

How is this tested?

N/A — metadata only, no runtime changes.