Add MongoDB storage library and update acceptance tests with deterministic stubs

- Created StellaOps.Notify.Storage.Mongo project with initial configuration.
- Added expected output files for acceptance tests (at1.txt to at10.txt).
- Added fixture input files for acceptance tests (at1 to at10).
- Created input and signature files for test cases fc1 to fc5.

docs/onboarding/dev-quickstart.md

@@ -55,6 +55,23 @@ Starter issues to grab on day 1 (all offline-friendly):
 
 UI note: Console remains in flux; focus on backend determinism first, then follow UI sprints 0209/0215 for micro-interactions and proof-linked VEX updates.
 
+## 3. Environment & DB matrix
+
+- MongoDB: 6.0.12 (pin in `inputs.lock`).
+- Optional Postgres slices: see sprint 340x series; keep read-only in dev until instructed.
+- Offline feeds: `offline-cache-2025-11-30` (scanner, advisories, VEX).
+- Timezone: `TZ=UTC` for all tests and tooling.
+
+## 4. Secrets & signing
+
+- Store short-lived signing keys in `~/.stellaops/keys` (gitignored); never commit secrets.
+- Use DSSE for pack manifests and fixtures; include signer IDs.
+- For Rekor: use mirrored bundle (no live log writes); verify receipts offline.
+
+## 5. Contribution checklist
+
+See `docs/onboarding/contribution-checklist.md` for the minimal gates (docs trailer, seeds, inputs.lock, DSSE, secrets).
+
 Helpful docs:
 
 - `docs/modules/platform/*` – protocols (DSSE envelopes, lattice terms, trust receipts).