remove the create_repo agent tool

mara ruled on #4849 (c88934): "remove create_repo tool". The tool ran in
hive-c0re with the hive's core token, so it only ever worked for agents
on the hive that runs the forge.

Removed:

- the create_repo MCP tool and CreateRepoArgs (hive-agent-mcp)
- wire variants Request::CreateRepo and Response::RepoCreated
  (hive-core-agent-sock)
- hive-c0re's handle_create_repo, its valid_repo_name check and the
  dispatch arm
- forge::create_agent_repo and apply_operator_branch_protection, which
  had no other caller, plus AGENTS_ORG and OPERATORS_TEAM, whose only
  users they were
- the tool's docs (docs/tools/forge.md repo management, docs/turn-loop/
  mcp.md, the conventions tool-group table) and the doc comments that
  named it (hive-sock-client's response timeout, ensure_repo_creation_
  disabled, the security doc's merge-gate bullet)

ToolGroup::Forge is kept with no tools, the same way b88a5b24 kept
Lifecycle, so existing meta/capabilities.json grants still parse.

Forge state is untouched: existing agents/* repos keep their collaborators
and operators-team branch protection. The swarm-controller's own
create_repo (config-org repos) is a different path and is unchanged.

Closes #4849

docs/trust-boundary/security.md

@@ -125,10 +125,10 @@ checkpoints**, not about sandboxing the agent from its own tools:
   **human (the operator) merges the PR**, keeping a person in the loop on the
   highest-value action. On the **internal forge this is technically enforced,
   not just convention**: agents can't create repos (`max_repo_creation = 0`),
-  so every repo is `core`-created with branch protection **on by default** —
-  merges restricted to the operators team + a required operators-team approval
-  (`apply_operator_branch_protection` / the config-repo equivalent) — and an
-  agent (a write collaborator, not a repo admin) can neither change those
+  and `main` on an `agent-configs/<name>` repo carries swarm-controller's
+  branch protection: merge allowlisted to the `operators` team, with one
+  approval from it. Existing `agents/<repo>` repos carry the same merge gate.
+  An agent (a write collaborator, not a repo admin) can neither change those
   settings nor merge its own PR. It's **not** set up for external VCS (GitHub
   etc.), though — there, operator-merge is process + accepted risk, not a
   technical control.