Establish dev workflow (feature-branch→main) and pull Gitea bootstrap earlier

- Add CONTRIBUTING.md: interim local-git rules (no work on main; every change
  a feature branch merged into main) and the target state (Gitea-hosted repos,
  GitOps source cutover, CI/CD via Gitea Actions running the Kyverno gate +
  Chainsaw conformance Job, PR merge gate). Note the self-hosted-git
  bootstrapping circularity + off-cluster mirror/backup mitigation.
- AGENTS.md: add section 12 (version-control workflow).
- docs/05: source-of-truth migration (local bootstrap -> self-hosted Gitea).
- docs/13 + roadmap: reprioritize Gitea as bootstrapping-critical, onboarded
  early (WS-APPS step 2); nextcloud onboarding split out.
- task-board: T-6.1a (gitea early), T-6.1b (source+CI migration to Gitea).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

README.md

@@ -61,6 +61,7 @@ GitOps configuration promotes to production with only environment-scoped overrid
 home-network-k3s/
 ├── README.md                     ← you are here
 ├── AGENTS.md                     ← rules of engagement for AI agents
+├── CONTRIBUTING.md               ← git workflow (feature branches → main; Gitea/CI-CD to come)
 ├── GLOSSARY.md                   ← shared vocabulary (read to avoid ambiguity)
 ├── docs/                         ← the specification, by domain
 │   ├── 00-vision-and-scope.md