agent-credential-broker: fork of vaultwarden-agent-broker + OpenBao backend

Mechanical rename (package agent_credential_broker; entry points
agent-broker{,-mcp,-plugin}; env AGENT_BROKER_*; paths agent-broker;
agent-browser credential provider 'broker'; MCP server name agent-broker).

New backend 'openbao': stdlib-only HTTP client for OpenBao/Vault KV v2
(approle login or token file/env; metadata-only ref index; lazy re-login
on 401/403; token renewal). bitwarden-cli backend kept as transition.

agent_credential_broker/errors.py

@@ -0,0 +1,76 @@
+"""Stable machine-readable error codes returned by the broker.
+
+Every failure crosses the socket/MCP boundary as {code, message} only --
+never with credential material attached.
+"""
+
+
+class BrokerError(Exception):
+    """Error that is safe to surface to the agent (no secret material)."""
+
+    def __init__(self, code, message):
+        super().__init__(message)
+        self.code = code
+        self.message = message
+
+    def to_dict(self):
+        return {"code": self.code, "message": self.message}
+
+
+def locked():
+    return BrokerError("BROKER_LOCKED", "broker is locked; run 'agent-broker unlock'")
+
+
+def unconfigured(detail="missing API client credentials"):
+    return BrokerError(
+        "UNCONFIGURED",
+        f"broker not configured: {detail}",
+    )
+
+
+def not_found(ref):
+    return BrokerError("NOT_FOUND", f"no credential with ref {ref!r}")
+
+
+def ambiguous(ref, count):
+    return BrokerError("AMBIGUOUS_REF", f"ref {ref!r} matches {count} items; fix agent.ref duplicates")
+
+
+def adapter_denied(ref, adapter):
+    return BrokerError("ADAPTER_DENIED", f"credential {ref!r} is not allowed for adapter {adapter!r}")
+
+
+def origin_denied(ref, target):
+    return BrokerError("ORIGIN_DENIED", f"credential {ref!r} is not authorized for target {target!r}")
+
+
+def denied(message):
+    return BrokerError("DENIED", message)
+
+
+def invalid_argument(message):
+    return BrokerError("DENIED_INVALID_ARGUMENT", message)
+
+
+def confirmation_required(ticket=None):
+    err = BrokerError("CONFIRMATION_REQUIRED", "operation requires interactive confirmation")
+    err.ticket = ticket
+    return err
+
+
+def unsafe_tool_integration(tool):
+    return BrokerError(
+        "UNSAFE_TOOL_INTEGRATION",
+        f"{tool}: no credential-provider or safe injection path exists; refusing integration",
+    )
+
+
+def backend_error(message):
+    """Vault backend failure (e.g. `bw` subprocess). Never silently
+    falls back to another backend."""
+    return BrokerError("VAULT_BACKEND_ERROR", message)
+
+
+def internal(message):
+    """Server-side fault. Message must already be secret-free."""
+    return BrokerError("INTERNAL_ERROR", message)