feat(supervise): non-blocking MCP — pending carries proposal id + check-proposal poll tool

Closes #412.

The supervise MCP server blocked the agent's tool call polling for the
operator's decision, and on timeout returned `status: pending` with no
proposal id and no way to poll a specific proposal — so the only way to
learn a late decision was to re-propose (a duplicate).

- `handle_tools_call` pending timeout now returns the `proposal_id` and
  points the agent at `check-proposal`.
- New `check-proposal` MCP tool: non-blocking status lookup by proposal id
  (pending | approved | modified | rejected | unknown). Reuses the queue's
  FileNotFoundError semantics; archives a decided proposal exactly like the
  synchronous path, so a pending proposal stays visible to the operator
  until it's both decided and polled.
- `TOOL_CHECK_PROPOSAL` constant, re-exported from supervise; kept out of
  TOOLS since it never becomes a Proposal.tool.

Enforcement is unchanged — the tools only propose policy; the egress proxy
and git-gate still enforce — so returning early opens no hole. Follow-ups
(git-gate reject-requeue, backpressure, notifications, web console) are in
the PRD.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YBCHap11yGAKuKfsehNPaD

bot_bottle/supervise.py

@@ -47,6 +47,7 @@ from .supervise_types import (
     STATUS_MODIFIED,
     STATUS_REJECTED,
     TOOLS,
+    TOOL_CHECK_PROPOSAL,
     TOOL_EGRESS_ALLOW,
     TOOL_EGRESS_BLOCK,
     TOOL_EGRESS_TOKEN_ALLOW,
@@ -263,6 +264,7 @@ __all__ = [
     "TOOLS",
     "EGRESS_FORWARD_PROXY",
     "EGRESS_INTROSPECT_URL",
+    "TOOL_CHECK_PROPOSAL",
     "TOOL_EGRESS_ALLOW",
     "TOOL_EGRESS_BLOCK",
     "TOOL_GITLEAKS_ALLOW",