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/integration/git-provider-api.md

@@ -0,0 +1,66 @@
+---
+checkId: check.integration.git
+plugin: stellaops.doctor.integration
+severity: warn
+tags: [connectivity, git, scm]
+---
+# Git Provider API
+
+## What It Checks
+Resolves the configured Git provider URL from `Git:Url`, `Scm:Url`, `GitHub:Url`, `GitLab:Url`, or `Gitea:Url`. Auto-detects the provider type (GitHub, GitLab, Gitea, Bitbucket, Azure DevOps) from the URL and sends an HTTP GET to the corresponding API endpoint (e.g., GitHub -> `api.github.com`, GitLab -> `/api/v4/version`, Gitea -> `/api/v1/version`, Bitbucket -> `/rest/api/1.0/application-properties`). The check **passes** if the response is 2xx, 401, or 403 (reachable even if auth is needed), **warns** on other non-error status codes, and **fails** on connection errors or exceptions.
+
+## Why It Matters
+Git provider connectivity is essential for source-code scanning, SBOM ingestion, webhook event reception, and commit-status reporting. A misconfigured or unreachable Git URL silently breaks SCM-triggered workflows and prevents evidence collection from source repositories.
+
+## Common Causes
+- Git provider URL is incorrect or has a trailing-path typo
+- Network connectivity issues or DNS failure
+- Git provider service is down or undergoing maintenance
+- Provider uses a non-standard API path
+
+## How to Fix
+
+### Docker Compose
+```bash
+# Check current Git URL
+grep 'GIT__URL\|SCM__URL\|GITHUB__URL' .env
+
+# Test from inside the network
+docker compose exec gateway curl -sv https://git.example.com/api/v4/version
+
+# Update the URL
+echo 'Git__Url=https://git.example.com' >> .env
+docker compose restart gateway
+```
+
+### Bare Metal / systemd
+```bash
+# Verify configuration
+cat /etc/stellaops/appsettings.Production.json | jq '.Git'
+
+# Test connectivity
+curl -v https://git.example.com/api/v4/version
+
+# Fix the URL
+sudo nano /etc/stellaops/appsettings.Production.json
+sudo systemctl restart stellaops-platform
+```
+
+### Kubernetes / Helm
+```yaml
+# values.yaml
+git:
+  url: https://git.example.com
+```
+```bash
+helm upgrade stellaops ./chart -f values.yaml
+```
+
+## Verification
+```
+stella doctor run --check check.integration.git
+```
+
+## Related Checks
+- `check.integration.ci.system` -- CI systems often share the same Git host
+- `check.integration.webhooks` -- webhook endpoints receive events from Git providers