security: T-4 decision-ledger hash chain + STRIDE claim/evidence CI gate

[noisyloop]

Summary

Follow-up to #39 (merged: STRIDE audit truth-up + E-5/T-5 fixes). That PR was merged before this final commit landed, so this PR carries the remaining two pieces the audit called for: build T-4 and add the claim/evidence CI gate. Single commit, clean on top of merged main.

T-4 — decision-ledger tamper-evident hash chain (was ❌ aspirational → now ✅)

The decision ledger previously had per-entry hashes only, so deleting or reordering entries was undetectable — a real hole in a non-repudiation/compliance-critical artifact.

• Every LedgerEntry now carries previousHash, chained from GENESIS.
• New streaming verifyChain() (readline, no OOM — consistent with D-4) fails closed on content tampering, deletion, or reordering, reporting the breaking index.
• Chain head bootstraps from disk in initialize() so appends keep chaining across restarts.
• Legacy pre-chain entries (no previousHash) remain verifiable: per-entry hash still checked, chain resumes from them — no migration needed.
• docs/STRIDE.md T-4 corrected ❌ → ✅ 2026-05-18.

Structural fix — STRIDE claim/evidence CI gate

scripts/check-stride-claims.mjs (npm run check:stride, wired into security.yml):

• Every finding in the STRIDE table must have a manifest entry (a new finding can't be added without a conscious decision).
• A ✅ finding must be test-backed (an attack-path test that references the id) or explicitly audit-attested with a code anchor.
• A non-✅ finding must declare resolved:false — the doc and the manifest cannot silently diverge.

This is the durable fix for the D-6 class of "marked done, not built." It already caught a real drift during development (T-4 left ❌ in the doc after being implemented → gate failed until reconciled).

Test plan

• npm test — 126/126, 10 suites
• npm run check:stride — PASS (29 findings: 4 test-backed, 19 audit-attested debt, 6 not-resolved)
• New suite decision-ledger-chain — clean-verify, content tamper, deletion, reorder, legacy back-compat
• Cherry-picked clean onto post-security: STRIDE verification audit — fixes (E-5, T-5, T-4), truth-up, claim/evidence CI gate #39 main; typecheck clean (only the pre-existing stale non-test tests/security/model-guard.ts errors remain, untouched)