feat(#2377): forge-webhook-triggered config-PR merge flow

Replace the request_merge_config_pr MCP tool with a Forgejo
pull_request webhook on the agent-configs org. Agents now open a
config PR normally; hive-c0re auto-queues the MergeConfigPr approval
from the webhook event — no extra tool call needed.

Changes:
- dashboard/webhook.rs: add POST /webhook/config-pr handler
  - parses Forgejo pull_request payload (opened/synchronize)
  - strips agent-configs/<agent> prefix to extract agent name
  - calls submit_merge_config_pr → queues dashboard approval card
  - always 200 to prevent Forgejo retries; errors logged at warn
- dashboard/mod.rs: wire /webhook/config-pr route
- forge/mod.rs: add ensure_config_pr_webhook() — idempotent org-level
  hook registration on agent-configs at startup; CONFIG_ORG now
  pub(crate) for webhook handler
- main.rs: call ensure_config_pr_webhook alongside knowledge webhook
- socket_server/config_approvals.rs: drop handle_request_merge_config_pr;
  make submit_merge_config_pr pub(crate) for webhook handler
- socket_server/mod.rs: re-export submit_merge_config_pr; drop dispatch arm
- hive-sh4re/src/lib.rs: remove AgentRequest::RequestMergeConfigPr
  wire type; drop from ToolGroup::Approvals tool list
- hive-ag3nt/src/mcp/: drop request_merge_config_pr tool + args struct
- docs: update approvals.md (webhook trigger), conventions.md (tool
  group), agent-hierarchy.md, tools/lifecycle.md

Hardening from #2375-merge-config-pr-hardening branch preserved:
- pr_is_open check at queue time (rejects closed/merged PRs)
- atomic fetched_sha INSERT via submit_kind(fetched_sha: Some(&sha))

Approve-handler machinery unchanged (run_merge_config_pr,
ff_push_to_main, fetch_pr_head_into_applied, mark_pr_merged).

hive-c0re/src/socket_server/mod.rs

@@ -25,12 +25,12 @@ mod lifecycle_handlers;
 mod reminders;
 mod schedules;
 
+pub(crate) use config_approvals::{submit_init_config, submit_merge_config_pr};
 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_merge_config_pr,
-    handle_request_update_meta_inputs,
+    handle_request_apply_commit, handle_request_init_config, handle_request_update_meta_inputs,
 };
 use lifecycle_handlers::{
     handle_kill, handle_list_descendants, handle_restart, handle_start, handle_update,
@@ -579,23 +579,6 @@ 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 } => {