feat(harness): add policy-aware token trimming

[senamakel]

Summary

• add an order-preserving token-budget trim primitive with a caller-supplied estimator
• optionally preserve system instructions even when they exceed the nominal budget
• optionally drop leading orphan tool results after older assistant calls are evicted
• retain the existing TrimStrategy::MaxTokens behavior for compatibility

TinyAgents already assigns native images a flat context weight. The estimator hook lets hosts extend that accounting to provider-specific inline image encodings without baking those encodings into the crate.

Validation

• cargo test token_budget_trim --lib
• cargo clippy --all-targets -- -D warnings
• cargo fmt --check

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📥 Commits

Reviewing files that changed from the base of the PR and between 2583fcc and e84ee1c.

📒 Files selected for processing (3)

• src/harness/summarization/mod.rs
• src/harness/summarization/test.rs
• src/harness/summarization/types.rs

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