Enforce the tool-call contract, asking servers only where it works

The executor ended a milestone on any reply without a tool call, and
nothing held a model to the offered tools. Measured on the 4070:
tool_choice "required" is enforced by vLLM (local-fast) and Anthropic,
but qwen3-14b (local-coder) under it cannot stop and writes until the
output ceiling, a known model bug. So enforcement is asked for per
model, and the loop holds the contract itself everywhere.

- The catalog reads supports_tool_choice from LiteLLM's model_info;
  BudgetedClient sends tool_choice only to models declared true, as
  "required" on the normalized route and {"type": "any"} on the
  Anthropic passthrough, and only when tools are offered
- The executor gains an explicit finish tool, the only way to end a
  milestone. Finishing with nothing changed is refused once; a prose
  reply gets one nudge, then escalates; a call to a tool not offered
  counts toward failure_limit, like a failed command; calls after
  finish are recorded as not run
- Clients record the model that actually answered. ModelFacts.answered
  recognises a proxy fallback, which once credited Claude's answers to
  a local model that could not call tools at all
- The context probe reports a fallback instead of trusting it; rerun,
  both local models now refuse over-long prompts, as they should
- The capability matrix checks every answer came from the model asked,
  that models call only offered tools, and that a declared
  supports_tool_choice is true
- Spec A15 and the tuning guide say how enforcement is requested,
  declared and backed up

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

scripts/probe_context.py

@@ -45,7 +45,8 @@ def main() -> int:
         return 2
 
     catalog = ModelCatalog.discover(config.llm_base_url, config.llm_token)
-    declared = catalog.facts(args.alias).max_input_tokens
+    facts = catalog.facts(args.alias)
+    declared = facts.max_input_tokens
     if declared is None:
         print(
             f"LiteLLM declares no max_input_tokens for {args.alias}; set one first",
@@ -65,7 +66,7 @@ def main() -> int:
         headers={"Authorization": f"Bearer {config.llm_token}"},
         timeout=600,
     ) as http:
-        result = probe(http, args.alias, declared)
+        result = probe(http, args.alias, declared, facts.backing)
     print(f"{args.alias}: {result.message}")
     return 0 if result.verdict in ("matches", "refuses", "declared_conservative") else 1