Expose reranker score in MCP/REST explain output

[Kyzcreig]

Summary

• Adds optional explain: true support to the MCP query tool and HTTP REST /query//search path.
• Exposes a top-level rerankScore per result when explain output is requested.
• Keeps default responses unchanged: no explain, no rerankScore unless explain: true is supplied.
• Maps disabled-rerank sentinel behavior to null instead of exposing 0 as a semantic relevance score.

Why

QMD already computes a pure semantic reranker score inside HybridQueryExplain.rerankScore, but callers at the MCP/HTTP boundary only receive the blended display score. That blended score includes RRF/position signal, so it is useful for ranking but not for applying a semantic relevance floor.

This exposes the existing pure reranker signal without changing the default response shape.

Test plan

• bun install
• bun run test:types
• CI=true bunx vitest run test/mcp.test.ts --reporter=verbose --testTimeout 60000
• bun run build
• NODE_NO_WARNINGS=1 bun run test:unit

Note: bun run test:unit without NODE_NO_WARNINGS=1 failed locally only on pre-existing test/cli.test.ts stderr-empty assertions due to Node DEP0205 module.register() deprecation warnings; rerunning with warnings suppressed produced 883 pass / 80 skip / 0 fail.

[Kyzcreig]

Hi! This PR has been open for about 37 days and hasn't had a look from anyone yet. It's not a draft and currently shows as mergeable, so it should be ready for a first pass whenever someone has a moment.

No decision needed from me right now — just flagging it for visibility so it doesn't keep sitting in the queue. Happy to rebase, split it up, or add more context if that would make reviewing easier. Thanks!

[PowderAddicts]

Dear tobi/qmd, We would like to acknowledge that we have received your request and a ticket has been created. A support representative will be reviewing your request and will send you a personal response.(usually within 24-48 hours). Thank you for your patience.

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