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/process/standup-summary.sample.md

@@ -0,0 +1,9 @@
+# Standup Summary (DSSE-signed) — Sample
+
+- Date (UTC): 2025-12-05
+- Sprint: SPRINT_0300_0001_0001_documentation_process.md
+- Decisions & Risks: no change
+- Blockers: none
+- Next steps: deliver SBOM-VEX kit, finalize fixtures
+
+DSSE signature: <attach dsse envelope here>