fix: address PR #75 review findings

MAJOR fixes:
- ci.yml: Add fork protection (github.event.pull_request.head.repo.full_name check)
  to prevent secret exfiltration from malicious fork PRs. Added security comment
  explaining the trust model for this private repo.
- ci.yml: Set GITHUB_SERVER_URL to explicit Gitea URL instead of github.server_url
  since reviews are posted to Gitea, not GitHub.
- release.yml: Set GITEA_URL explicitly to https://gitea.weiker.me since releases
  are created on Gitea.
- action.yml: Change gitea-url default from empty (fallback to github.server_url)
  to explicit https://gitea.weiker.me. Update all internal uses to rely on this
  default rather than falling back to server_url.

MINOR fixes:
- action.yml: Update header comment to reflect dual-platform (Gitea Actions +
  GitHub Actions) support.
- action.yml: Fix repo input description to say it defaults to rodin/review-bot
  for version lookup, matching the actual code behavior.
- pr-ready-gate.yml: Add comments explaining why Gitea URL is hardcoded (intentional:
  we update Gitea PR from GitHub mirror) and noting the PR number matching assumption.

All findings from sonnet-review, gpt-review, and security-review addressed.

.github/workflows/release.yml

@@ -37,8 +37,9 @@ jobs:
           GITEA_TOKEN: ${{ secrets.RELEASE_TOKEN }}
         run: |
           VERSION=${GITHUB_REF_NAME}
-          GITEA_URL="${{ github.server_url }}"
-          REPO="${{ github.repository }}"
+          # Releases are created on Gitea, not GitHub - use explicit Gitea URL
+          GITEA_URL="https://gitea.weiker.me"
+          REPO="rodin/review-bot"
 
           # Create release (or find existing one for this tag)
           HTTP_CODE=$(curl -s -o /tmp/release_response.json -w "%{http_code}" -X POST \