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>

tests/integration/test_gitea_live.py

@@ -5,29 +5,17 @@ Cleans up after itself (closes the PR, deletes the branch) so repeated runs
 do not silt up the sandbox.
 """
 
-import os
 import uuid
 from pathlib import Path
 
 import pytest
 
-from nisse_dev.envfile import load_env_file
+from nisse_dev.config import AgentConfig
 from nisse_dev.git import GitError, GitRepo, authenticated_url
 from nisse_dev.gitea import GiteaClient, pull_request_reconciler
 from nisse_dev.state import Intent
 
-load_env_file(Path(__file__).resolve().parents[2] / ".env")
-
-SENTINEL = "NISSE_GITEA_URL"
-ALSO_REQUIRED = ("NISSE_GITEA_TOKEN", "NISSE_GITEA_TEST_REPO")
-
-pytestmark = [
-    pytest.mark.integration,
-    pytest.mark.skipif(
-        not os.environ.get(SENTINEL),
-        reason=f"no live Gitea configured ({SENTINEL} unset)",
-    ),
-]
+pytestmark = pytest.mark.integration
 
 
 def intent_for(branch: str) -> Intent:
@@ -41,16 +29,13 @@ def intent_for(branch: str) -> Intent:
     )
 
 
-def test_push_open_pr_and_reconcile(tmp_path: Path) -> None:
-    missing = [var for var in ALSO_REQUIRED if not os.environ.get(var)]
-    assert not missing, (
-        f"{SENTINEL} is set, so the live Gitea path is expected to work, "
-        f"but these are missing: {', '.join(missing)}"
-    )
-    base_url = os.environ[SENTINEL]
-    token = os.environ["NISSE_GITEA_TOKEN"]
-    repo_path = os.environ["NISSE_GITEA_TEST_REPO"]
-    client = GiteaClient.from_env()
+def test_push_open_pr_and_reconcile(
+    tmp_path: Path, live_config: AgentConfig, gitea: GiteaClient
+) -> None:
+    base_url = live_config.gitea_url
+    token = live_config.gitea_token
+    repo_path = live_config.gitea_repo  # the sandbox
+    client = gitea
 
     account = client.whoami()
     assert "testing" in account, (