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/nisse-test.yaml

@@ -0,0 +1,40 @@
+# Agent config for the live integration tests, aimed at test resources: the
+# testing identities, the dedicated test room, and the sandbox repo. Never
+# point it at production.
+#
+# Secrets are ${…} references. CI passes them as environment variables;
+# locally they come from .env at the repository root. A missing one fails
+# the live tests that need it, rather than skipping them, so a broken
+# credential cannot hide as a skip.
+
+# Unused by the live tests, which work in pytest's temporary directories;
+# present because every agent config has them.
+state_db: /tmp/nisse-test/state.db
+workspace: /tmp/nisse-test/work
+project: nisse-dev-testing
+
+llm:
+  base_url: https://litellm.dongar.net
+  token: ${NISSE_LLM_TOKEN}
+
+# The live tests exercise the gated loop and the routes over a real proxy,
+# not local-model competence (measured separately by test_model_capability),
+# so every role uses a model that holds the tool-call contract.
+roles:
+  executor: claude-haiku-4-5
+  planner: claude-haiku-4-5
+  delegate: claude-haiku-4-5
+  consultant: claude-haiku-4-5
+
+matrix:
+  homeserver: https://matrix.dongar.net
+  user_id: "@nisse-dev-testing:dongar.net"
+  token: ${NISSE_MATRIX_TOKEN}
+  room_id: "!UxpXhTlQsFIeNZwkgv:dongar.net"   # the test room, never production
+  operators: ["@dongar:dongar.net"]
+
+gitea:
+  url: https://gitea.dongar.net
+  token: ${NISSE_GITEA_TOKEN}
+  repo: dongar/nisse-dev-testing-sandbox      # the sandbox, never a real project
+  operators: [dongar]