Fit every model request to its context window

Every turn re-sent the whole transcript with nothing checking it against
the model's window. On a local model that fails silently: Ollama drops
the start of an over-long prompt, which is where the system prompt and
the plan are. Measured on the Jetson's llama3.2, cut at 4096 tokens with
no error.

- The model catalog reads max_input_tokens and max_output_tokens from
  LiteLLM's model_info, and startup refuses a role on a model with no
  declared window
- BudgetedClient wraps both routes. It fits the canonical events before
  rendering: older tool results cut to head and tail, then the oldest
  turns dropped with a note, keeping calls with their results; a request
  that still cannot fit fails with ContextOverflow and is never sent.
  The output ceiling is the smaller of config and the model's limit
- Tokens are estimated from characters, pessimistically, and the
  estimate is corrected from reported prompt sizes, only ever upward
- The executor takes its scope from the log: the latest plan, which is
  the approved milestone plan, goes in its system prompt, and it sees
  only events after it
- Tool output keeps its head and, mostly, its tail, so a test runner's
  failure summary survives truncation
- tuning.context.keep_recent_turns and safety_margin are new knobs
- scripts/probe_context.py checks a declared window against what the
  server does; it confirmed llama3.2 cuts at the declared 4096
- The tuning guide covers the budget, declaring a local model's limits,
  and the probe

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

nisse-dev/tests/test_config.py

@@ -297,3 +297,12 @@ def test_sibling_paths_are_fine(tmp_path: Path) -> None:
         {**SECRETS, "NISSE_ENV": str(tmp_path / ".env")},
     )
     assert config.workspace.name == "project"
+
+
+def test_context_tuning_is_read_and_checked() -> None:
+    config = build(tuning={"context": {"keep_recent_turns": 4, "safety_margin": 0.2}})
+    assert config.tuning.context.keep_recent_turns == 4
+    assert config.tuning.context.safety_margin == 0.2
+    assert build().tuning.context.keep_recent_turns == 6
+    with pytest.raises(ConfigError, match="safety_margin: must be less than 1"):
+        build(tuning={"context": {"safety_margin": 1}})