feat(hive-forge): add pr-merge verb (#1670)

Adds `hive-forge pr-merge <n> [--method merge|rebase] [--keep-branch]
[--force]` wrapping POST /repos/{owner}/{repo}/pulls/{n}/merge, so agents on
the peer-review-and-merge workflow have a CLI path instead of the raw API.

- Methods: merge (default) | rebase. Squash is intentionally not offered.
- Deletes the head branch after merge unless --keep-branch.
- Safe by default: refuses unless the PR is mergeable, CI is not red, and no
  review's current verdict requests changes (latest-per-reviewer wins, so a
  later approval clears an earlier request-changes). --force overrides and
  also sets Forgejo's force_merge.
- New client helper post_no_content for the 200-empty-body merge response.

hive-forge/src/client.rs

@@ -229,6 +229,29 @@ impl Client {
         Ok(())
     }
 
+    /// POST a JSON body to `<api>/<path>` for an endpoint that returns a
+    /// 2xx with an empty body (so there is nothing to decode). Used by
+    /// `pr-merge` — Forgejo's merge endpoint answers `200 OK` with no body
+    /// on success and a non-2xx (e.g. `405`) when the PR is not mergeable.
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if the request fails to send (transport/network
+    /// error) or the server responds with a non-2xx status (the response
+    /// body is included in the error).
+    pub fn post_no_content<B: Serialize>(&self, path: &str, body: &B) -> Result<()> {
+        let url = format!("{}{}", self.api(), path);
+        let resp = self
+            .http
+            .post(&url)
+            .header(CONTENT_TYPE, "application/json")
+            .json(body)
+            .send()
+            .context("POST")?;
+        check_status(resp, &format!("POST {url}"))?;
+        Ok(())
+    }
+
     /// DELETE `<api>/<path>`. Optional JSON body for endpoints that
     /// need it (Forgejo's subscription unwatch uses bodyless DELETE).
     pub fn delete(&self, path: &str, body: Option<&Value>) -> Result<()> {