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.

Makefile

@@ -0,0 +1,16 @@
+.PHONY: test lint install service-install
+
+test:
+	python3 -m unittest discover -s tests -v
+
+lint:
+	python3 -m compileall -q agent_credential_broker tests
+
+install:
+	pip3 install --user -e .
+
+service-install:
+	install -D -m 644 systemd/agent-broker.service \
+		~/.config/systemd/user/agent-broker.service
+	systemctl --user daemon-reload
+	@echo "Now: systemctl --user enable --now agent-broker"