feat: auto-promote non-repair candidate skills

[RerankerGuo]

Fixes #1941.

Summary

• Auto-promote untried non-repair candidate skills once their eta reaches the retrieval threshold.
• Keep repair-origin candidates on the trial path so repairs still require evidence before activation.
• Run a skill lifecycle tick during pipeline flush after pending skill writes are drained.

Validation

• npm test -- --run tests/unit/skill/lifecycle.test.ts
• npm run lint
• Attempted: npm test -- --run tests/unit/skill/subscriber.test.ts -t "lifecycleTick"; blocked before assertions because better-sqlite3 native bindings are unavailable in this local Node 18 / compiler environment.
• Attempted: make format; blocked because poetry is not installed in this environment.

[RerankerGuo]

Closing this draft because current main already contains the candidate skill lifecycle promotion path, including the lifecycle tick after skill flush.