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>

src/__Libraries/StellaOps.Doctor.Plugins.Security/Checks/RateLimitingCheck.cs

@@ -63,7 +63,8 @@ public sealed class RateLimitingCheck : IDoctorCheck
                 })
                 .WithCauses("Rate limiting explicitly disabled in configuration")
                 .WithRemediation(r => r
-                    .AddManualStep(1, "Enable rate limiting", "Set RateLimiting:Enabled to true"))
+                    .AddManualStep(1, "Enable rate limiting", "Set RateLimiting:Enabled to true")
+                    .WithRunbookUrl("docs/doctor/articles/security/security-ratelimit.md"))
                 .WithVerification("stella doctor --check check.security.ratelimit")
                 .Build());
         }