fix(stdio): bound embedQuery and fast-fail during background reindex

[aeneasr]

Summary

Hardens the fix-hanging short-circuit (merged in PR #161-ish) with three follow-ups uncovered during review:

• Bound embedQuery (defense-in-depth) — Mirror the existing defaultSearchTimeout on idx.Search. Without an outer bound, the embedder's 10-minute HTTP client timeout surfaces as an effective hang in Claude Code on any embedder pathology (cold-start GPU, network drop, server crash mid-request). New defaultEmbedTimeout = 20s.
• Fast-fail embed + preserve stale results — The merged fix returned warning-only whenever StaleWarning was set, throwing away results from the (still-readable) DB. Now we give the embed a short window (defaultStaleEmbedTimeout = 3s); if it succeeds, return stale-but-real results plus the warning; if it times out, fall back to warning-only.
• Drop brittle "10 tool calls" advice — That count is wildly under-counted for large repos like cloud where a first-time index runs 10+ minutes. New wording points at grep/glob/find and notes that big repos take longer. Also consolidated four duplicate copies into a single staleIndexWarning constant.

Test plan

• TestHandleSemanticSearch_EmbedQueryBounded — defense-in-depth path: blocking embedder, no StaleWarning, returns with embed query error within ~500ms (not 10 minutes)
• TestHandleSemanticSearch_StaleWarningFastEmbedReturnsStaleResults — fast embed + flock held: returns stale results from the DB plus the warning
• TestHandleSemanticSearch_StaleWarningShortCircuits — updated for new fast-fail behavior (embed is attempted, gets cancelled within staleEmbedTimeout)
• TestStaleIndexWarning_NoBrittleToolCallCount — asserts the new wording
• go test ./... PASS (~2 min)
• golangci-lint run ./... zero issues
• go vet ./... clean
• End-to-end against the original cloud repo reproduction: lumen purge ~/workspace/go/cloud && lumen index ~/workspace/go/cloud (background), then semantic_search via stdio while indexer holds the flock — debug.log shows stale embed fast-fail: returning warning-only with timeout=3s and the call returns immediately. No 10-minute hang.

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 49d50e8d-33a7-4056-8eb7-40ee04c768c5

📥 Commits

Reviewing files that changed from the base of the PR and between 89555d0 and afbc6c9.

📒 Files selected for processing (3)

• cmd/stdio.go
• cmd/stdio_concurrency_test.go
• cmd/stdio_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-hanging-followup

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