fix(skills): default cache hints and preserve legacy wire responses - #1

Summary

Follow-up to the cache-hint review on upstream draft #2818. This PR targets feat/ext-skills-phase-1 in this fork so the fix can be reviewed before updating the upstream draft.

• Default omitted cacheHint to { ttlMs: 0, cacheScope: 'private' } for both skills/list and skills/get.
• Include hints only on modern protocol revisions (2026-07-28 onward). Resolve the server's negotiated era at request time; legacy responses omit both fields, even when hints are configured.
• Keep the core cache-method allowlist and wire codecs unchanged. Document the extension's era-specific behavior.
• Replace the omission assertion with a shared five-case modern matrix and compact legacy wire assertions for both operations, covering all five supported legacy revisions and modern-looking metadata on a legacy connection.

Validation

• Server suite: 515 tests passed, including 33 focused Skills tests.
• Client suite: 896 tests passed.
• Core suite: 18 tests passed.
• Repository-wide pre-push checks passed: pnpm build:all, pnpm typecheck:all, and pnpm lint:all (including snippet sync).
• git diff --check passed.

The first sandboxed client-suite run timed out in localhost HTTP/SSE tests; rerunning with localhost access passed all 896 tests.

Implemented and tested with Codex assistance.