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-ag3nt/src/mcp/args.rs

@@ -222,6 +222,17 @@ pub struct RequestApplyCommitArgs {
     pub description: Option<String>,
 }
 
+#[derive(Debug, serde::Deserialize, schemars::JsonSchema)]
+pub struct RequestMergeConfigPrArgs {
+    /// Logical agent name whose `agent-configs/<agent>` forge repo holds the PR.
+    pub agent: String,
+    /// Open PR index on the `agent-configs/<agent>` repo.
+    pub pr_number: u64,
+    /// Optional description shown on the dashboard approval card.
+    #[serde(default)]
+    pub description: Option<String>,
+}
+
 #[derive(Debug, serde::Deserialize, schemars::JsonSchema)]
 pub struct UpdateMetaInputsArgs {
     /// Flake input names to update (e.g. `["bitburner-agent", "nixpkgs"]`).