feat(docs): Add comprehensive documentation for Vexer, Vulnerability Explorer, and Zastava modules

- Introduced AGENTS.md, README.md, TASKS.md, and implementation_plan.md for Vexer, detailing mission, responsibilities, key components, and operational notes.
- Established similar documentation structure for Vulnerability Explorer and Zastava modules, including their respective workflows, integrations, and observability notes.
- Created risk scoring profiles documentation outlining the core workflow, factor model, governance, and deliverables.
- Ensured all modules adhere to the Aggregation-Only Contract and maintain determinism and provenance in outputs.

docs/modules/graph/README.md

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+# StellaOps Graph
+
+Graph module (upcoming) will power graph-indexed queries for SBOM relationships, lineage, and blast-radius analysis.
+
+## Responsibilities
+- Model SBOM and advisory entities as a navigable graph.
+- Provide APIs for dependency impact, provenance chains, and reachability analysis.
+- Integrate with Scheduler/Policy for graph-driven re-evaluation.
+- Expose tooling for offline explorers.
+
+### Domain highlights (Epic 5)
+- **Nodes:** artifacts/images, SBOM components, packages/versions, files/paths, licences, advisories, VEX statements, provenance attestations, policy versions.
+- **Edges:** `depends_on`, `contains`, `built_from`, `declared_in`, `affected_by`, `vex_exempts`, `governs_with`, `produced_by`, each timestamped and tenant-scoped.
+- **Overlays:** policy verdict overlays, VEX consensus, runtime telemetry, and export-ready snapshots with diff support.
+- **Queries:** reachability (`impact(graph, advisory)`), blast radius (`reverseDepends(component)`), provenance timeline, saved query library with semantic zoom for Console.
+
+## Key components
+- Planned services documented in implementation plan (to be delivered).
+
+## Integrations & dependencies
+- SBOM Service / Cartographer for data ingestion.
+- Policy & CLI for query surfaces.
+
+## Operational notes
+- Pending — see implementation plan for staged milestones.
+
+## Backlog references
+- DOCS-GRAPH-24-003 (architecture index) and SCHED-MODELS-21-001 tasks.
+
+## Epic alignment
+- **Epic 5 – SBOM Graph Explorer:** deliver graph indexer, API, Console explorer, saved queries, overlays, and exports.