Make Codex skills project-aware

[pandec]

Summary

• discover Codex skills for the active project's working directory instead of relying only on the server startup directory
• add a small project-scoped RPC keyed by provider instance and cwd, while retaining the provider snapshot as a fallback
• use the project-scoped result in the composer $ menu and document that minimizing upstream merge cost takes precedence over broader refactors for private-fork changes

Root cause

The provider snapshot was populated from the T3 server process working directory and reused globally by every composer. Starting T3 from one repository could therefore expose that repository's local skills, but conversations for other projects would not refresh discovery against their own cwd.

Verification

• vp test run apps/server/src/provider/providerSkills.test.ts
• vp run typecheck
• vp check
• isolated T3 browser verification: opened the $ menu for this worktree and confirmed both repo-local sync-upstream and test-t3-app skills appear alongside global skills