fix: improve search quality and make vectors opt-in for 1.0.1

[lightsifter]

Summary

• Improve semantic candidate ranking and cold search execution while retaining raw similarity and clear coverage.
• Make local vector search explicitly opt-in for Pi, OMP and MCP. Ordinary exact search remains model-free; disabled concept/hybrid requests fail clearly.
• Improve request recovery and compact model-facing diagnostics; clarify actual memory/latency costs and Jev behavior in both READMEs.

Verification

• bun run check:local: 505 passed, 3 conditional skips, 0 failed.
• bun run test:mcp-hosts: Claude, Codex and Kimi stdio flows passed.
• bun run pack:check: npm 1.0.1 dry run passed.
• bun run test:concept: 8 passed with the real local model.
• Compiled Bun worker real-model integration: 4 passed.

Related: #94. Close after publication and installation verification.