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

### 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//codekb//` (`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** (#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: #716 (stale-result propagation, the reuse target), #414 (incremental/versioned artifact history — adjacent), #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.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of the [project license](https://github.com/awslabs/aidlc-workflows/blob/main/LICENSE).

Start in core/tools/aidlc-lib.ts at codekbDir and the codekbScopeFingerprint/codekbSourceFingerprint seam, then trace the reverse-engineering reuse gate and core/tools/aidlc-validity.ts. Follow QUESTION_ANSWERED handling in the harness and add tests at the appropriate tiers. Done means changed-file deltas and provenance-checked answer reuse are reported, full rescan bypasses reuse, and the stated acceptance scenarios pass.

Written by the indexing model from the issue text.