Move configuration to a YAML file, and make the tuning knobs configurable

The thresholds Part H says to tune on the real inference hardware were
hardcoded, so every adjustment would have been a PR. They now live in a
`tuning:` section of a YAML config file, which also replaces flat
environment variables as the home for structure (#32).

- The file (NISSE_CONFIG, default ./nisse-dev.yaml) carries structure
  and holds no secrets: credentials are ${VAR} references resolved from
  the environment, and a reference to an unset variable is an error
- The environment still wins: every existing NISSE_* variable overrides
  its key, and the file is optional, so .env and CI work unchanged
- Validation stays fail-fast and reports every problem at once; unknown
  keys are errors, so a misspelt knob fails at startup instead of
  silently doing nothing
- The config file joins the state store and .env in the check that
  refuses to start when they sit inside the workspace clone
- Tuning covers the executor's turn and failure limits, the consult
  budget, the delegation caps, the command timeout and the tool output
  limit, which Workspace now takes as a setting; the defaults are the
  values the agent already ran with
- llm.max_output_tokens reaches both LLM clients from the config

docs/Tuning-Guide.md explains the knobs for someone learning local
model tuning: how weights, quantization and the KV cache spend VRAM,
what each knob does and what goes wrong at either extreme, and an order
to tune in. nisse-dev.example.yaml documents every key and is checked
by a test so it cannot drift.

Closes #32

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

nisse-dev/src/nisse_dev/main.py

@@ -20,7 +20,7 @@ from nisse_dev.envfile import load_env_file
 def main() -> int:
     load_env_file(Path(os.environ.get("NISSE_ENV", ".env")).expanduser())
     try:
-        config = AgentConfig.from_env()
+        config = AgentConfig.load()
     except ConfigError as exc:
         # Fail here rather than twenty minutes into a milestone on the one
         # call that needed the missing value.