feat(#125): rename GITEA_URL to VCS_URL with deprecated fallback

- Add --vcs-url flag as primary (reads VCS_URL env var)
- Keep --gitea-url and GITEA_URL as deprecated fallbacks with warnings
- Update action.yml: rename gitea-url input to vcs-url, pass VCS_URL to binary
- Update ci.yml: use VCS_URL env var in Run review step
- Update integration tests: INTEGRATION_GITEA_URL -> INTEGRATION_VCS_URL
- Update README: --vcs-url / VCS_URL with fallback note in env var table

Backward compat: existing GITEA_URL users get a deprecation warning and
continue to work unchanged until they migrate to VCS_URL.

README.md

@@ -299,7 +299,7 @@ All flags have environment variable equivalents:
 
 | Flag | Env Var |
 |------|---------|
-| `--gitea-url` | `GITEA_URL` |
+| `--vcs-url` | `VCS_URL` (fallback: `GITEA_URL`) |
 | `--repo` | `GITEA_REPO` |
 | `--pr` | `PR_NUMBER` |
 | `--reviewer-token` | `REVIEWER_TOKEN` |