feat(prd-0081): reprovision CA, git-gate, and egress tokens on gateway bring-up

On a Firecracker gateway cold boot, reconcile every live agent VM against
the fresh gateway: push the new mitmproxy CA into each agent's trust store,
re-provision git-gate repos/creds from the persisted upstreams snapshot,
and restore egress tokens. Per-bottle failures are logged and skipped so
one unreachable VM does not block the rest.

New modules / changes:
- backend/firecracker/reconcile.py: attach_bottled_agents_to_gateway,
  _push_ca, _reprovision_git_gate, _guest_ip_from_config
- git_gate/provision.py: write upstreams.json after key provisioning so
  the bring-up reconcile can reconstruct the upstream table without the
  manifest
- backend/firecracker/infra.py: call attach_bottled_agents_to_gateway in
  the cold-boot branch of ensure_running()
- backend/base.py: no-op default on BottleBackend
- backend/firecracker/consolidated_launch.py: remove superseded
  _reprovision_running_bottles / _guest_ip_from_config
- orchestrator: OrchestratorCore.update_agent_secret + POST
  /bottles/<id>/secret + client.update_agent_secret (single-secret
  in-place update, the reusable primitive from the 2026-07-26 hotfix)
- tests: 15 new tests in test_firecracker_reconcile.py; updated
  test_firecracker_infra.py cold-boot cases; stale FC reprovision tests
  removed from test_backend_secret_reprovision.py

Closes #516.

bot_bottle/orchestrator/service.py

@@ -379,6 +379,30 @@ class OrchestratorCore:
         self._tokens[bottle_id] = decrypted
         return True
 
+    def update_agent_secret(
+        self, bottle_id: str, name: str, value: str, env_var_secret: str,
+    ) -> bool:
+        """Update ONE egress token for a known bottle in place — the
+        single-secret form of ``reprovision_from_secret``.
+
+        Sets the in-memory token AND upserts the single re-encrypted row under
+        *env_var_secret* (the same key the rest of the rows are encrypted with, so
+        the whole set stays decryptable by a later ``reprovision_from_secret``),
+        leaving every other token untouched. Returns False if the bottle is
+        unknown.
+
+        Unlike ``reprovision_from_secret`` (which restores the values captured at
+        launch), this pushes a *caller-supplied* value — used to refresh a
+        short-lived host credential (e.g. the Codex access token) into a
+        still-running bottle without a relaunch."""
+        from .store.secret_store import encrypt_value
+        if self.registry.get(bottle_id) is None:
+            return False
+        self._tokens.setdefault(bottle_id, {})[name] = value
+        self.registry.store_agent_secret(
+            bottle_id, name, encrypt_value(env_var_secret, value))
+        return True
+
     # --- consolidated gateway ----------------------------------------------
 
     def gateway_status(self) -> dict[str, object]: