feat(#2346): request_merge_config_pr — submission path for the PR-based config flow

MergeConfigPr approvals had a fully-implemented approve handler
(run_merge_config_pr, ff_push_to_main, mark_pr_merged) and dashboard
display, but no way to submit one.  An agent with the `approvals` tool
group calling request_merge_config_pr(agent, pr_number) is the missing
piece.

What this adds:
- RequestMergeConfigPr variant in hive-sh4re AgentRequest + ToolGroup::Approvals
- submit_merge_config_pr: fetches PR head sha (the drift-gate reviewed sha),
  queues a MergeConfigPr row, sets fetched_sha, emits approval_added with
  pr_number so the dashboard card links to the forge PR
- handle_request_merge_config_pr: topology (require_descendant) +
  tool-group (require_group(approvals)) guards before submit
- socket_server/mod.rs: dispatch arm for RequestMergeConfigPr
- hive-ag3nt MCP tool: request_merge_config_pr with full description
- docs/tools/lifecycle.md: documents the new tool + boundary table row

Unlike submit_apply_commit, no flake pre-flight at submission time (eval-
verify happens at approval time inside run_merge_config_pr, same as the
rest of the merge pipeline).  Applied repo must already exist (guard added
with a clear error message pointing at request_apply_commit for first-spawn).

hive-c0re/src/socket_server/mod.rs

@@ -29,7 +29,8 @@ pub(crate) use schedules::filter_ghost_schedule_targets;
 pub use schedules::schedule_to_wire_public;
 
 use config_approvals::{
-    handle_request_apply_commit, handle_request_init_config, handle_request_update_meta_inputs,
+    handle_request_apply_commit, handle_request_init_config, handle_request_merge_config_pr,
+    handle_request_update_meta_inputs,
 };
 use lifecycle_handlers::{
     handle_kill, handle_list_descendants, handle_restart, handle_start, handle_update,
@@ -578,6 +579,23 @@ async fn dispatch(req: &AgentRequest, agent: &str, coord: &Arc<Coordinator>) ->
             )
             .await
         }
+        AgentRequest::RequestMergeConfigPr {
+            agent: target_agent,
+            pr_number,
+            description,
+        } => {
+            if let Some(err) = require_group(agent, "approvals", "request merge_config_pr") {
+                return err;
+            }
+            handle_request_merge_config_pr(
+                coord,
+                agent,
+                target_agent,
+                *pr_number,
+                description.as_deref(),
+            )
+            .await
+        }
         // Agent-state queries: own subtree is free; other agents + the
         // hive-wide `"*"` sweep require `QueryAgentState`.
         AgentRequest::GetLooseEnds { agent: target } => {