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>

docs/Architecture-Build-Spec.md

@@ -668,7 +668,13 @@ subsystem.
 
 ### A15. Local Model Accommodations
 
-- **Constrained decoding for tool-call JSON.**
+- **Constrained decoding for tool-call JSON.** Requested with `tool_choice`
+  (`required`), and only of models whose LiteLLM entry declares
+  `supports_tool_choice`: enforcement is the model server's, and asking one that
+  can't honour it can be worse than not asking. Where it isn't available, the
+  loop enforces the contract itself: an explicit `finish` tool ends the work, a
+  prose reply is a violation, and a call to a tool not offered counts as a
+  failure (A12).
 - **Reduced tool schema for the executor role:** fewer tools, flatter arguments, no
   deeply nested optionals. Maintain **two tool manifests keyed on role**, rendered at
   request time like everything else.