fix: close ungated CLI merge bypasses in review_pr.py and merge_pr.py (#16)

Reviewer found the MCP merge surface is gated but two local CLI scripts remain
ungated merge paths that LLM automations in this project have been using.
Close them (Option B — minimal safe fix; full gated CLI merge left to a
follow-up):

- review_pr.py: `--merge` now fails closed BEFORE any API call with a clear
  message directing callers to the gated `gitea_merge_pr` MCP workflow. The
  review-only path is unchanged. The merge execution block was removed.
- merge_pr.py: main() is now a fail-closed no-op — reads no credentials and
  makes no merge API call; prints that merge is only available via the gated
  workflow.
- README: the `review_pr.py` row and Quick Examples no longer advertise a CLI
  `--merge` path; added an audit-logging clarification that #16 returns
  structured gate/merge results but does not add durable audit logging, which
  is tracked by #18.

Tests updated/added:
- test_review_pr.py: `--merge` fails closed with no API call; message points to
  the gated workflow.
- test_merge_pr.py: merge fails closed with no API call, even with
  --force/--do/--title/--message; message points to the gated workflow.
- test_mcp_server.py: README no longer advertises the ungated CLI merge example.

The gated MCP `gitea_merge_pr` is unchanged and still gated; `gitea_review_pr`
still fails closed on merge=True; `gitea_submit_pr_review` still cannot merge.
No secrets, auth headers, raw env, or credential paths are exposed. No
Jenkins/Ops/GlitchTip/Release/deploy/CI behavior added. #17/#18 not started.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -180,6 +180,9 @@ Notes:
   can inspect which runtime it is talking to before deciding to act.
 - See [`docs/gitea-execution-profiles.md`](docs/gitea-execution-profiles.md) for
   the full profile model.
+- **Audit logging:** #16 returns structured gate/merge results but does not add
+  durable audit logging. Durable audit logging for Gitea MCP mutating actions is
+  tracked by #18.
 </details>
 
 <details>
@@ -204,7 +207,7 @@ The MCP tools can also be used as standalone CLI scripts:
 | `create_issue.py`   | Create an issue (`--remote`, `--title`, `--body`, `--body-file`)   |
 | `create_pr.py`      | Open a Pull Request (`--remote`, `--title`, `--head`, `--base`)    |
 | `edit_pr.py`        | Edit a Pull Request (`--title`, `--body`, `--body-file`, etc.)      |
-| `review_pr.py`      | Review and sign-off on a pull request (with optional merge)        |
+| `review_pr.py`      | Review/sign-off on a pull request (`--merge` is disabled — fails closed; merge only via gated `gitea_merge_pr`) |
 | `close_issue.py`    | Close a specific issue                                             |
 | `mark_issue.py`     | Claim/release an issue via `status:in-progress` label              |
 | `manage_labels.py`  | Create label set and apply label mappings (`--dry` to preview)     |
@@ -225,8 +228,9 @@ The MCP tools can also be used as standalone CLI scripts:
 # Edit a PR's description or title
 ./edit_pr.py 155 --body "Updated description wording"
 
-# Review and approve a PR, then automatically merge it
-./review_pr.py --pr-number 12 --event APPROVE --body "Approved" --merge
+# Review and approve a PR (review only — CLI merge is disabled; use the
+# gated gitea_merge_pr MCP workflow to merge)
+./review_pr.py --pr-number 12 --event APPROVE --body "Approved"
 
 # Close issue #5
 ./close_issue.py 5