chore: move .github folder to .gitea for Gitea compatibility

Gitea prefers .gitea/ISSUE_TEMPLATE/ and .gitea/workflows/ over the
GitHub-convention .github/ equivalents. Moves all issue templates and
workflow files to the Gitea-native paths and updates CLAUDE.md references.

Cosign certificate identity paths in release/rollback workflows are
intentionally left unchanged — they reference the signing identity from
prior workflow runs and will need a separate update when the CI signing
infrastructure migrates.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

CLAUDE.md

@@ -158,7 +158,7 @@ Comments must be **timeless** — no references to this conversation, refactorin
 
 **Config:** `Dockerfile`, `apps/worker/configs/`, `apps/worker/prompts/`, `tsconfig.base.json` (shared compiler options), `turbo.json`, `biome.json`
 
-**CI/CD:** `.github/workflows/ci.yml` (type-check, lint, build & push images to GHCR), `.github/workflows/release.yml` (Docker Hub push + GitHub release, manual dispatch)
+**CI/CD:** `.gitea/workflows/ci.yml` (type-check, lint, build & push images to GHCR), `.gitea/workflows/release.yml` (Docker Hub push + GitHub release, manual dispatch)
 
 ## Package Installation