fix: harden knowledge base phase-1 index safety

[knqiufan]

Summary

Closes #17.

Implements the knowledge-base audit's phase-one remediation:

• versioned copy-on-write vector index activation
• durable KB jobs, cancellation/waiting, and cleanup retries
• actual active-index reconciliation for statistics and chat selection
• PyInstaller RAG dependencies and frozen smoke coverage
• audit report remediation status and implementation log

Verification

• pytest -q — 614 passed
• ruff check misaka/
• mypy misaka/
• pip check
• source RAG smoke (sqlite-vec load/write/search + BM25 fusion)

The PR CI additionally builds the Windows frozen artifact and runs Misaka.exe --rag-smoke.