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/conventions.md

@@ -325,7 +325,7 @@ binary flavor.
 | `inbox` | `get_loose_ends`, `cancel_loose_end`, `remind`, `request_next_turn` |
 | `execution` | vestigial — `mcp__bash__run` / `mcp__bash__status` are always available unconditionally via `extraMcpServers`; this group's entries expand to non-existent `mcp__hyperhive__run` / `mcp__hyperhive__status` and have no effect. See `docs/tools/bash.md`. |
 | `lifecycle` | `kill`, `start`, `restart`, `update` *(privileged)* |
-| `approvals` | `request_init_config`, `request_apply_commit`, `request_merge_config_pr`, `request_update_meta_inputs` *(privileged)* |
+| `approvals` | `request_init_config`, `request_apply_commit`, `request_update_meta_inputs` *(privileged)* |
 | `scheduling` | `request_schedule_prompt`, `fire_schedule_now`, `cancel_schedule`, `edit_schedule`, `list_schedules` *(privileged)* |
 | `diagnostics` | `get_logs` *(privileged)* |