feat: respect provider context_budget_cap for cost-optimal compaction

[sadlilas]

Summary

Reads context_budget_cap from ProviderInfo.defaults and caps the compaction budget at that threshold. This lets providers with tiered pricing (Anthropic, Gemini) signal their cost-optimal context limit without affecting providers with flat pricing (OpenAI, Ollama).

Why

The previous approach (PR #5, fraction-based) applied a blanket budget = context_window * 0.15 to ALL providers when the context window exceeded 200k. This penalized OpenAI (flat pricing, 400k window) for an Anthropic/Gemini-specific pricing concern:

Provider + Model	context_window	Fraction budget (PR #5)	Provider-cap budget (this PR)
Anthropic 1M	1,000,000	~146k	~196k (capped)
Gemini	1,048,576	~153k	~196k (capped)
OpenAI GPT-5	400,000	~56k (terrible)	~364k (full window)
Copilot gpt-5.1	264,000	~36k (terrible)	~228k (full window)

What changes

In _calculate_budget(), after computing the raw budget from context_window - reserved_output - safety_margin:

budget_cap = defaults.get("context_budget_cap")
if budget_cap is not None:
    budget = min(budget, budget_cap - safety_margin)

Applied in both code paths:

1. get_model_info() path -- reads cap from provider.get_info().defaults
2. get_info().defaults path -- reads cap from the same defaults dict already in scope

Providers that don't set context_budget_cap are unaffected -- the standard formula applies unchanged.

Design

This is the consumer side of the provider-driven budget cap. See amplifier-support#57 analysis for the full rationale.

• "Follow the Data" -- pricing knowledge originates in the provider and flows to the consumer
• "Compose Before You Create" -- uses the existing defaults dict, no new protocols or kernel fields
• Zero kernel changes -- convention in modules, not kernel contract

Related PRs

• provider-anthropic#27: Emits context_budget_cap: 200_000 when 1M context is active
• provider-gemini: Emits context_budget_cap: 200_000 (all models have the pricing cliff)

Supersedes

• PR feat: smart budget fraction for extended context windows #5 (fraction-based approach) -- will be closed once this approach is approved

Related issues

• microsoft-amplifier/amplifier-support#57

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