feat(pi): add experience and skill generation tools

[wutongyuonce]

Summary

Adds the missing Pi entry points for generating reviewable Experience and Skill candidates.

• pc_experience_generate -> generate_experience
• pc_skill_generate -> generate_skill

Refs #1338

Plan and final design

The original plan was to complete the next Pi capability gap after artifact inspection and structured work continuity, while keeping candidate governance separate.

The final implementation follows that plan:

1. pc_experience_generate accepts exact Source and Artifact references plus optional target and reason.
2. pc_skill_generate accepts the same evidence model and requires the direct provenance origin: experience, source, or usage.
3. Both tools route through the existing Scope-aware operation path. The request schemas match the OpenAPI contracts, including the combined maximum of 32 evidence references.
4. Both operations are durable writes: they require interactive confirmation, refuse writes without a UI, inject the resolved Scope, and reject secret-bearing payloads.
5. Generation returns a candidate for human review. It does not approve, publish, install, activate, or execute the resulting Experience or Skill. Candidate decisions remain outside the Pi tool surface.
6. The project-context Skill, capability manifest, bilingual Pi documentation, and focused tests describe and verify these boundaries.

This PR intentionally does not add candidate approval/rejection/revision or External Skills. Pi therefore gains the Experience/Skill generation part of the Full-profile contract, but this PR alone does not claim Full support.

Verification

• Pi package tests: 95 passed
• TypeScript typecheck: passed
• Integration manifest tests: 28 passed
• Generated manifest check: passed
• ty check: passed
• git diff --check: passed

Implementation and local verification were performed with Codex (GPT-5.6 Sol). An independent read-only review was performed in a separate Codex session using GPT-5.6 Luna with medium reasoning; all actionable findings were resolved before submission.