feat(retrieval): add manual BM25 query expansion and diagnostics

[rwmjhb]

Summary

This PR replaces #292 with a maintainer-owned branch.

It keeps the core retrieval ergonomics changes from the original PR while addressing the review feedback that was still open:

1. BM25 query expansion for manual / CLI retrieval only
2. Retrieval diagnostics on the retriever itself
3. CLI debug output for retrieval diagnostics, including failure paths from the ephemeral search retriever

Why

Users often search with colloquial phrases such as 挂了, 卡住, or 报错, while stored memories often contain more technical wording like crash, timeout, error, or exception.

The vector leg already helps semantically, but the BM25 leg still matters for exact-term boosting and mixed-language memory bases. This improves the explicit/manual lookup path while deliberately leaving auto-recall unchanged.

Scope and safety

• no change to auto-recall query behavior
• no change to vector query text
• query expansion stays limited to manual / CLI retrieval
• replacement branch drops the unrelated vllm rerank provider changes from feat(retrieval): add manual BM25 query expansion and diagnostics #292
• CLI failure diagnostics now come from the actual search retriever instance, not just context.retriever

Validation

node --test test/query-expander.test.mjs
node test/cli-smoke.mjs
node test/retriever-rerank-regression.mjs

Passed locally.

[AliceLJY]

LGTM — scope is clean, tests are thorough (774 lines), query expansion logic is solid.

One finding to confirm:

vectorOnlyRetrieval: recencyBoost no longer gated by decayEngine

In the original code, when decayEngine is present, applyRecencyBoost is skipped in the vector-only path. In the new code, applyRecencyBoost always runs regardless of decayEngine. This could cause double time-weighting (decay + recency) in the vector-only + decayEngine scenario.

bm25OnlyRetrieval and hybridRetrieval still gate recencyBoost behind !decayEngine, so this is an inconsistency in vectorOnlyRetrieval only.

If intentional, a one-line comment explaining why would be helpful. If not, restore the if (!this.decayEngine) guard.

Minor: formatRetrievalDiagnosticsLines in cli.ts uses an inline type that's a subset of RetrievalDiagnostics — consider importing the interface directly to avoid future type drift.