feat: add Atomic Chat as OpenAI-compatible local provider

Register atomic_chat in the provider registry with default base URL
http://localhost:1337/v1, schema field, docs, and config tests.

tests/cli/test_commands.py

@@ -371,6 +371,28 @@ def test_config_accepts_lm_studio_without_api_key_and_uses_default_localhost_api
     assert config.get_api_base() == "http://localhost:1234/v1"
 
 
+def test_config_accepts_atomic_chat_without_api_key_and_uses_default_localhost_api_base():
+    config = Config.model_validate(
+        {
+            "agents": {
+                "defaults": {
+                    "provider": "atomic_chat",
+                    "model": "local-model",
+                }
+            },
+            "providers": {
+                "atomicChat": {
+                    "apiKey": None,
+                }
+            },
+        }
+    )
+
+    assert config.get_provider_name() == "atomic_chat"
+    assert config.get_api_key() is None
+    assert config.get_api_base() == "http://localhost:1337/v1"
+
+
 def test_find_by_name_accepts_camel_case_and_hyphen_aliases():
     assert find_by_name("volcengineCodingPlan") is not None
     assert find_by_name("volcengineCodingPlan").name == "volcengine_coding_plan"
@@ -378,6 +400,8 @@ def test_find_by_name_accepts_camel_case_and_hyphen_aliases():
     assert find_by_name("github-copilot").name == "github_copilot"
     assert find_by_name("longcat") is not None
     assert find_by_name("longcat").name == "longcat"
+    assert find_by_name("atomic-chat") is not None
+    assert find_by_name("atomic-chat").name == "atomic_chat"
 
 
 def test_config_explicit_longcat_provider_resolves_provider_name():