fix(core): clarify agent context budgets

[cbusillo]

Summary

• advertise context_budget_tokens as an integer tool-schema field and return actionable guidance for bad budget values
• reuse the context-file budget guidance across parse and budget failures
• add broader UTF-8 exec stream regression coverage for partial prune and 4-byte overlap cases

Validation

• cargo test --manifest-path code-rs/Cargo.toml -p code-core context_budget -- --nocapture
• cargo test --manifest-path code-rs/Cargo.toml -p code-core exec_stream -- --nocapture
• ./build-fast.sh