Update AGENTS.md files across multiple modules to standardize task status update instructions and introduce a new document for Secret Leak Detection operations.

- Modified task status update instructions in AGENTS.md files to refer to corresponding sprint files as `/docs/implplan/SPRINT_*.md` instead of `docs/implplan/SPRINTS.md`.
- Added a comprehensive document for Secret Leak Detection operations detailing scope, prerequisites, rule bundle lifecycle, enabling the analyzer, policy patterns, observability, troubleshooting, and references.

src/Scanner/StellaOps.Scanner.WebService/AGENTS.md

@@ -21,7 +21,7 @@ Operate the Scanner WebService API, orchestrating scan requests, queue interacti
 - `docs/modules/scheduler/architecture.md` (rescan interactions)
 
 ## Working Agreement
-1. **Status updates**: change task state to `DOING`/`DONE` in both `docs/implplan/SPRINTS.md` and local `TASKS.md` when you start/finish work.
+1. **Status updates**: change task state to `DOING`/`DONE` in both sprint file `/docs/implplan/SPRINT_*.md` and local `TASKS.md` when you start/finish work.
 2. **Contract-first changes**: update OpenAPI specs/test fixtures when APIs change; coordinate with CLI/UI consumers.
 3. **Surface integration**: centralise cache/env/secret access via Surface libraries; run validators before executing handlers.
 4. **Determinism**: queue IDs, report manifests, and export metadata must remain stable; avoid wall-clock usage in response payloads.