Make the config file the only home for settings

With no production instance yet, there is no reason to carry two ways
of configuring the agent. Every environment variable that duplicated a
config key is gone; the environment now supplies only the secrets the
file references by ${…}, plus NISSE_CONFIG and NISSE_ENV to find the
files.

- AgentConfig loads from a file (load, from_file) or a parsed tree
  (from_tree, for tests); a missing file is an error naming the example
- The from_env constructors on LiteLLMClient, AnthropicPassthroughClient,
  GiteaClient, MatrixConfig and RoleMap are removed, with the env
  reading for max output tokens; clients take their settings from config
- The live integration tests share a committed agent config,
  tests/integration/nisse-test.yaml, aimed at test resources, and one
  conftest that loads it and builds clients. NISSE_LIVE_PLANNER_MODEL
  becomes its planner role; NISSE_CAPABILITY_MODELS stays as a per-run
  switch
- A missing secret now fails the live tests instead of skipping them
- CI passes only the three tokens
- .env.example is secrets only; spec A5 says config comes from the file

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

nisse-dev/tests/test_dev_main.py

@@ -2,25 +2,23 @@ from pathlib import Path
 
 import pytest
 
-from nisse_dev.config import REQUIRED
 from nisse_dev.main import main
 
 
-def test_main_refuses_to_start_without_configuration(
+def test_main_refuses_to_start_without_a_config_file(
     monkeypatch: pytest.MonkeyPatch,
     capsys: pytest.CaptureFixture[str],
     tmp_path: Path,
 ) -> None:
-    # Clear the real variables rather than swapping os.environ: from_env binds
-    # the environment object as a default argument, so a replacement would be
-    # ignored. And point NISSE_ENV at a file that does not exist, because the
-    # working directory's .env holds live credentials.
-    for name in REQUIRED:
-        monkeypatch.delenv(name, raising=False)
+    # Run from an empty directory, and point NISSE_ENV at a file that does not
+    # exist: the repository's .env holds live credentials, and a test must
+    # never be one cwd away from starting a real agent.
+    monkeypatch.chdir(tmp_path)
+    monkeypatch.delenv("NISSE_CONFIG", raising=False)
     monkeypatch.setenv("NISSE_ENV", str(tmp_path / "absent.env"))
 
     assert main() == 2
 
     err = capsys.readouterr().err
     assert "cannot start" in err
-    assert "NISSE_LLM_TOKEN" in err
+    assert "nisse-dev.example.yaml" in err