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/lifecycle/mod.rs

@@ -8,9 +8,8 @@ mod setup;
 mod tests;
 
 pub use git::{
-    git, git_authed, git_command, git_command_authed, git_delete_ref, git_is_ancestor,
-    git_read_tree_reset, git_rev_parse, git_tag, git_tag_annotated, git_update_ref,
-    git_update_ref_cas,
+    git, git_authed, git_command, git_command_authed, git_is_ancestor, git_read_tree_reset,
+    git_rev_parse, git_tag, git_update_ref, git_update_ref_cas,
 };
 pub use host_config::write_dropins;
 pub use setup::{