feat: warm re-scan / no re-interrogation on iteration (reuse CodeKB + prior answers)

[SaRedfiche]

Problem

On any iteration over an existing workspace/intent, AI-DLC re-runs a cold Initialization scan and re-interrogates the user with questions they already answered. A customer reported a 45-minute re-scan that re-asked answered questions for what was ultimately an 11-line fix. This is the "reuse prior workspace understanding" half of the last-30% iteration gap.

What already exists (verified against source)

The code-understanding side is largely built and should be reused, not rebuilt:

• Durable space-level CodeKB store aidlc/spaces/<space>/codekb/<repo>/ (core/tools/aidlc-lib.ts codekbDir).
• Git-tree-hash freshness fingerprint (codekbScopeFingerprint / codekbSourceFingerprint) and a scope_version:1 freshness block in reverse-engineering-timestamp.md.
• A CURRENT/STALE/... verdict (codekb-scope-diff) + a reuse gate in the reverse-engineering stage (Reuse / Full rescan / Focused scan), so the --rescan full escape hatch effectively exists already as "Full rescan".
• propagateStageInvalidation (core/tools/aidlc-validity.ts) already marks downstream artifacts stale/needs-revalidation without resetting completed stages (feat: project and propagate stale stage results #716 mechanism).

The two real gaps

1. Per-file delta, not store-wide. The freshness verdict today is whole-scope CURRENT/STALE — one changed file flips the entire store STALE with no changed-file set, so there is no "re-scan only what changed". Add a delta computation (git diff of the store's recorded generation vs HEAD, on the existing fingerprint seam) that returns the changed-file set, and make the reverse-engineering developer brief accept it.
2. Answered questions are not persisted as reusable Q→A pairs at all. QUESTION_ANSWERED events are write-only audit rows read back only for within-attempt authorization — never to pre-fill a later run. A durable, provenance-stamped answer store + a pre-fill reader (invoked before a stage's question fences) is a from-scratch component. Surface "reused N prior answers"; a changed question or changed code must re-ask rather than silently reuse.

Acceptance

• A second workflow over the same repo/intent completes Initialization by scanning only changed files and reports the cache hit + delta.
• Questions answered in the first run are not re-asked; the run reports how many prior answers were reused.
• --rescan full bypasses the cache and re-scans everything.

Notes

• Foundational: unblocks the amend scope and batch-ingestion work.
• Related: feat: project and propagate stale stage results #716 (stale-result propagation, the reuse target), [Feature]: Support incremental, versioned artifact history across project iterations #414 (incremental/versioned artifact history — adjacent), [RFC]: conditional stage-entry context load on resume, and a report-then-clarify ordering (or gate-turn exemption) for clarifying questions #1094 (clarifying-question ordering — adjacent to the re-ask logic). Roadmap Goal 5 (cyclic flows) / Goal 6.
• Author in core//harness/; keep dist/ generated; add tests at the appropriate tiers.

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