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/vuln-explorer/README.md

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+# StellaOps Vulnerability Explorer
+
+Vulnerability Explorer delivers policy-aware triage, investigation, and reporting surfaces for effective findings.
+
+## Responsibilities
+- Present policy-evaluated findings with advisory, VEX, SBOM, and runtime context.
+- Capture triage workflow in an immutable findings ledger with role-based access.
+- Provide pivots, exports, and reports for auditors and operations teams.
+- Integrate explain traces, remediation notes, and offline bundles.
+
+## Key components
+- Findings Ledger service + API.
+- Console module and CLI verbs for triage workflows.
+- Export integrations for reports and evidence packages.
+
+## Integrations & dependencies
+- Policy Engine for effective findings streams.
+- Concelier/Excititor for evidence provenance.
+- Scheduler for remediation/verification jobs.
+- Notify for triage notifications.
+
+## Operational notes
+- Audit logging per Epic 6 requirements.
+- Offline-ready CSV/PDF exports with deterministic hashes.
+- Dashboards for MTTR and triage throughput.
+
+## Epic alignment
+- Epic 6: Vulnerability Explorer.
+- VULN stories tracked in ../../TASKS.md and src/VulnExplorer/**/TASKS.md.