ci: migrate from GitHub Actions to Gitea Actions

Move workflows to .gitea/workflows and adapt for git.farh.net:
- Push container images to git.farh.net instead of GHCR/Docker Hub
- Publish Helm chart as OCI artifact (no gh-pages, Gitea lacks Pages)
- Replace cosign keyless signing with key-based (COSIGN_PRIVATE_KEY/PASSWORD/PUBLIC_KEY)
- Swap @semantic-release/github for semantic-release-gitea
- Drop gh CLI from rollback workflow
- Use GITEA_TOKEN for registry auth and release creation
- Add Artifact Hub annotations to Chart.yaml
- Run on ubuntu-latest

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

charts/hightower/Chart.yaml

@@ -4,3 +4,24 @@ description: API-driven AI pentester built on Shannon, deployed as a service on
 type: application
 version: 0.1.1
 appVersion: "1.0.0"
+home: https://git.farh.net/farhoodlabs/trebuchet
+sources:
+  - https://git.farh.net/farhoodlabs/trebuchet
+maintainers:
+  - name: farhoodlabs
+    url: https://git.farh.net/farhoodlabs
+keywords:
+  - security
+  - pentesting
+  - ai
+  - kubernetes
+annotations:
+  artifacthub.io/license: AGPL-3.0
+  artifacthub.io/links: |
+    - name: source
+      url: https://git.farh.net/farhoodlabs/trebuchet
+  artifacthub.io/images: |
+    - name: worker
+      image: git.farh.net/farhoodlabs/trebuchet:latest
+    - name: api
+      image: git.farh.net/farhoodlabs/trebuchet-api:latest