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).

docs/agent-hierarchy.md

@@ -99,7 +99,6 @@ Once enforcement lands the rules collapse into:
 | `kill` / `start` / `restart` / `update` (any descendant)                | any ancestor                                                                            |
 | `request_init_config` (spawn a new child)                               | any agent, child added under self                                                       |
 | `request_apply_commit` (any descendant's config)                        | any ancestor                                                                            |
-| `request_merge_config_pr` (any descendant's forge config PR)            | any ancestor                                                                            |
 | `get_logs` (any descendant)                                             | any ancestor                                                                            |
 | moderate questions / reminders (cancel any open thread of a descendant) | any ancestor                                                                            |
 | `send` / `recv` routing                                                 | parent ↔ same-parent siblings ↔ self ↔ descendants; explicit allow-list for anyone else |