fix: allow non-AIR clients to opt into goals

[mrmans0n]

Summary

• let non-AIR clients opt into the goal extension with clientCapabilities._meta.goal: {}
• advertise and publish the provider-neutral goal metadata at top-level _meta.goal for those clients
• preserve the existing AIR goal wire shape and keep every other AIR extension gated as before

Tests

• npx vitest run --no-file-parallelism src/__tests__/CodexACPAgent/initialize.test.ts src/__tests__/CodexACPAgent/thread-goal-events.test.ts
• npm run typecheck
• npm test (1,212 passed, 33 skipped)

Fixes #582