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/backends/__init__.py

@@ -0,0 +1,70 @@
+"""Vault backend abstraction -- the ONLY layer that talks to the vault.
+
+    bitwarden-cli    official `bw` subprocesses against a
+                     Bitwarden-compatible server (transition backend).
+    openbao          OpenBao/Vault HTTP API (KV v2 + AppRole), stdlib
+                     urllib only -- the target backend for agent
+                     credentials.
+
+A failing backend surfaces as VAULT_BACKEND_ERROR; there is no fallback.
+
+The backend owns authentication, the unlocked session and vault reads.
+Everything else (policy, refs, adapters, sanitizer, MCP, unix socket)
+is backend-agnostic.
+"""
+
+from __future__ import annotations
+
+from ..errors import BrokerError
+
+
+class VaultBackend:
+    """Interface consumed by BrokerCore.
+
+    unlock(password)    authenticate + establish session (raises
+                        UNLOCK_FAILED on bad password, VAULT_BACKEND_ERROR
+                        otherwise)
+    lock()              invalidate session + drop in-memory material
+    list_metadata()     items with secret fields stripped (ref index)
+    get_item(item_id)   full normalized item, fetched on demand
+    """
+
+    name = "abstract"
+
+    def __init__(self, config):
+        self.config = config
+
+    def configured(self) -> bool:
+        raise NotImplementedError
+
+    def unlock(self, password: str) -> None:
+        raise NotImplementedError
+
+    def lock(self) -> None:
+        raise NotImplementedError
+
+    def logout(self) -> None:
+        """End the vault session AND the stored account login. Default
+        equals lock() for backends without a persistent login."""
+        self.lock()
+
+    def list_metadata(self) -> list[dict]:
+        raise NotImplementedError
+
+    def get_item(self, item_id: str) -> dict:
+        raise NotImplementedError
+
+
+def make_backend(config) -> VaultBackend:
+    name = config.vault_backend
+    if name == "bitwarden-cli":
+        from .bwcli import BitwardenCliBackend
+        return BitwardenCliBackend(config)
+    if name == "openbao":
+        from .openbao import OpenBaoBackend
+        return OpenBaoBackend(config)
+    raise BrokerError(
+        "UNCONFIGURED",
+        f"unknown vault_backend {name!r} "
+        "(supported: 'bitwarden-cli', 'openbao')",
+    )