feat(vault): rebuild chains.json via Gemini 3.1 Pro Preview — 373 curated chains

Replaced the 726 author-curated chains with 373 LLM-curated chains
generated bucket-by-bucket within (track, topic). Gemini was prompted
with the strict-progression + multi-chain constraints we agreed on:

  - Δ ∈ {1, 2} between consecutive members (prefer +1)
  - Up to 2-chain membership only for L1/L2 anchors
  - Single-topic, 2-6 members, no Δ=0 same-level pairs
  - Validated structurally on apply — vault check --strict passes

Sweep stats:
  - 44 calls to gemini-3.1-pro-preview (well under 250/day cap)
  - 313 (track, topic) buckets processed in ~80 minutes
  - 373 chains accepted (51% of legacy count, much higher per-chain
    quality after strict filter)
  - Level-Δ distribution: 949 strict +1 (93%), 73 +2 (7%) — 0 +0/+3+
  - Chain sizes: 26 size-2, 141 size-3, 128 size-4, 60 size-5, 18 size-6
  - 1,395 questions in chains (15% of corpus, vs ~20% before)
  - 54 of ~87 topics have at least 1 chain
  - 138 corpus gaps identified (gaps.proposed.json) — missing-rung
    questions that would complete chains; feeds future authoring pass

Why fewer chains than before is fine:
  - Old chains had a long tail with cos<0.65 (worse than random
    same-bucket pairs). LLM curation rejects those.
  - We trade quantity for pedagogical coherence.
  - The 138 gaps capture what was implicit in old chains via
    questions-that-shouldnt-have-been-paired; we make it explicit.

Files:
  - chains.json — applied (was backed up to chains.json.bak by
    apply_proposed_chains.py)
  - chains.proposed.json — kept for review/audit
  - gaps.proposed.json — authoring backlog
  - vault-manifest.json + corpus-summary.json — regenerated
  - corpus.json — gitignored (CI regenerates)

Validation: vault check --strict 0 failures, vault build clean,
playwright UI suite 13/13 pass.

interviews/staffml/src/data/vault-manifest.json

@@ -1,11 +1,11 @@
 {
   "releaseId": "dev",
-  "releaseHash": "ba3a0a7458d65073afc3f6381d86455fd526bd531106f258b108119bd79f8ca1",
+  "releaseHash": "937fb2c9db19d8a311a25f4fe6657371a18d920ce4d666784055b988e96bee41",
   "schemaVersion": "1",
   "policyVersion": "1",
-  "buildDate": "2026-04-30T12:45:57Z",
+  "buildDate": "2026-04-30T19:14:49Z",
   "questionCount": 9438,
-  "chainCount": 726,
+  "chainCount": 373,
   "conceptCount": 87,
   "trackDistribution": {
     "cloud": 4028,