config PRs: remove the hive's config-PR webhook, poll and core merge

An operator's merge on the forge deploys a config PR through
swarm-controller's DeployRequest{rev}. The hive-side path that queued a
MergeConfigPr approval and merged the PR as `core` goes:

- the `/webhook/config-pr` receiver, its HMAC secret, the WebhookRegister
  boot node and the org-hook registration; the hive vhost's `/webhook/`
  location
- the 5-minute config-PR poll
- ApprovalKind::MergeConfigPr, its dashboard card, and the deploy DAG it
  drove (DeployWindow, MergeVerify, DeployApply, FinalizeDeploy,
  DeployTail), with verify_commit, the two-phase meta deploy, rollback
  refs, the PR-failure comment and forge/pr_merge.rs
- `fetched_sha`, `sha_short`/`pr_number` on approval events, and
  `sha`/`tag` on HelperEvent::ApprovalResolved: only the merge path set
  them

`config_repo`, `merged_pr_for_commit` and `post_pr_comment` move to
forge/pr_comment.rs for the merged-rev deploy's refusal comment.
Approvals v5 drops stored `merge_config_pr` rows; a test reopens a v4
database holding them.

Closes #4850

hive-c0re/src/server.rs

@@ -424,7 +424,7 @@ async fn handle_set_resource_limits(
     // Goes through `meta::commit_resource_limits`, not the bare
     // `resource_limits::set_limits`: the write has to be staged +
     // committed under `META_LOCK` or it leaves the meta working tree
-    // dirty for the next `prepare_deploy` / `sync_agents` to trip over.
+    // dirty for the next `sync_agents` to trip over.
     crate::meta::commit_resource_limits(name.as_str(), &limits).await?;
 
     // Re-apply the drop-in straight away — same three lines as the job