feat(orchestrator): update a single egress secret in place (0081)

Add a single-secret counterpart to reprovision_from_secret's restore-all:
update ONE egress token for a running bottle without a relaunch, for
refreshing a short-lived host credential (e.g. the Codex access token)
whose launch-time snapshot has expired.

- registry_store.store_agent_secret: per-key upsert (delete+insert of the
  one row), the counterpart of store_agent_secrets' replace-all.
- OrchestratorCore.update_agent_secret: set the in-memory token AND upsert
  the re-encrypted row under the bottle's env_var_secret, leaving other
  tokens untouched.
- POST /bottles/<id>/secret (cli-only) + client.update_agent_secret.

Refs #510, #512

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/orchestrator/server.py

@@ -200,6 +200,31 @@ def dispatch(  # pylint: disable=too-many-return-statements,too-many-branches
             return 200, {"reprovisioned": True}
         return 404, {"error": "no stored secrets for this bottle"}
 
+    if (
+        method == "POST"
+        and route.startswith("/bottles/")
+        and route.endswith("/secret")
+    ):
+        # Update ONE egress token for a running bottle in place — the
+        # single-secret form of reprovision, for refreshing a short-lived host
+        # credential (e.g. the Codex access token) without a relaunch. cli-only
+        # (not in _GATEWAY_ROUTES): a bottle must never set its own tokens.
+        bottle_id = route[len("/bottles/") : -len("/secret")]
+        try:
+            data = _parse_json_object(body)
+        except ValueError as e:
+            return 400, {"error": f"invalid JSON: {e}"}
+        name = data.get("name")
+        value = data.get("value")
+        env_var_secret = data.get("env_var_secret")
+        if not all(isinstance(v, str) and v for v in (name, value, env_var_secret)):
+            return 400, {
+                "error": "name, value, and env_var_secret (non-empty strings) are required"
+            }
+        if orch.update_agent_secret(bottle_id, name, value, env_var_secret):
+            return 200, {"updated": True}
+        return 404, {"error": "no such bottle"}
+
     if method == "DELETE" and route.startswith("/bottles/"):
         bottle_id = route[len("/bottles/"):]
         if orch.teardown_bottle(bottle_id):