Doctor plugin checks: implement health check classes and documentation

Implement remediation-aware health checks across all Doctor plugin modules
(Agent, Attestor, Auth, BinaryAnalysis, Compliance, Crypto, Environment,
EvidenceLocker, Notify, Observability, Operations, Policy, Postgres, Release,
Scanner, Storage, Vex) and their backing library counterparts (AI, Attestation,
Authority, Core, Cryptography, Database, Docker, Integration, Notify,
Observability, Security, ServiceGraph, Sources, Verification).

Each check now emits structured remediation metadata (severity, category,
runbook links, and fix suggestions) consumed by the Doctor dashboard
remediation panel.

Also adds:
- docs/doctor/articles/ knowledge base for check explanations
- Advisory AI search seed and allowlist updates for doctor content
- Sprint plan for doctor checks documentation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/doctor/articles/_TEMPLATE.md

@@ -0,0 +1,47 @@
+---
+checkId: check.<plugin>.<name>
+plugin: stellaops.doctor.<plugin>
+severity: fail | warn | info
+tags: [tag1, tag2]
+---
+# <Check Name>
+
+## What It Checks
+<Exact condition tested. What thresholds trigger fail vs warn vs pass. What evidence is collected.>
+
+## Why It Matters
+<Business impact if this check fails in production. What breaks, what data is at risk, what users experience.>
+
+## Common Causes
+- Cause 1 (specific: exact misconfiguration, missing file, wrong env var)
+- Cause 2
+- Cause 3
+
+## How to Fix
+
+### Docker Compose
+```bash
+# Step-by-step commands for docker-compose deployments
+# Use exact env var names with __ separator
+# Reference exact file paths relative to devops/compose/
+```
+
+### Bare Metal / systemd
+```bash
+# Step-by-step commands for bare-metal / systemd deployments
+# Reference exact config file paths (e.g., /etc/stellaops/appsettings.json)
+```
+
+### Kubernetes / Helm
+```bash
+# Step-by-step commands for Kubernetes/Helm deployments
+# Reference exact Helm values, ConfigMap keys, Secret names
+```
+
+## Verification
+```bash
+stella doctor run --check check.<plugin>.<name>
+```
+
+## Related Checks
+- `check.related.id` - brief explanation of relationship