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/__init__.py

@@ -0,0 +1,9 @@
+"""agent-credential-broker: capability layer between agents and a vault.
+
+Invariant: agents may USE credentials through this broker, but can never
+RETRIEVE credential values. Every operation resolves a stable reference
+(``agent.ref`` custom field), enforces an item-scoped policy, executes a
+bounded adapter, and returns only sanitized metadata.
+"""
+
+__version__ = "0.1.0"