feat(acp): add env field to ACPAgentConfig for subprocess env injection (#1447)

Allow per-agent environment variables to be declared in config.yaml under
acp_agents.<name>.env. Values prefixed with $ are resolved from the host
environment at invocation time, consistent with other config fields.
Passes None to spawn_agent_process when env is empty so the subprocess
inherits the parent environment unchanged.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/test_acp_config.py

@@ -64,10 +64,36 @@ def test_load_acp_config_none_clears_agents():
 def test_acp_agent_config_defaults():
     cfg = ACPAgentConfig(command="my-agent", description="My agent")
     assert cfg.args == []
+    assert cfg.env == {}
     assert cfg.model is None
     assert cfg.auto_approve_permissions is False
 
 
+def test_acp_agent_config_env_literal():
+    cfg = ACPAgentConfig(command="my-agent", description="desc", env={"OPENAI_API_KEY": "sk-test"})
+    assert cfg.env == {"OPENAI_API_KEY": "sk-test"}
+
+
+def test_acp_agent_config_env_default_is_empty():
+    cfg = ACPAgentConfig(command="my-agent", description="desc")
+    assert cfg.env == {}
+
+
+def test_load_acp_config_preserves_env():
+    load_acp_config_from_dict(
+        {
+            "codex": {
+                "command": "codex-acp",
+                "args": [],
+                "description": "Codex CLI",
+                "env": {"OPENAI_API_KEY": "$OPENAI_API_KEY", "FOO": "bar"},
+            }
+        }
+    )
+    cfg = get_acp_agents()["codex"]
+    assert cfg.env == {"OPENAI_API_KEY": "$OPENAI_API_KEY", "FOO": "bar"}
+
+
 def test_acp_agent_config_with_model():
     cfg = ACPAgentConfig(command="my-agent", description="desc", model="claude-opus-4")
     assert cfg.model == "claude-opus-4"