* fix(agents): skip pre-prompt precheck when context engine owns compaction
When a context engine advertises ownsCompaction=true (e.g. lossless-claw),
skip the pre-prompt preemptive overflow precheck entirely. The engine
already manages the context budget through assemble() and its own
compaction lifecycle — the built-in precheck is redundant and causes
false-positive overflow errors for CJK-heavy sessions due to its
conservative token estimation formula.
Safety is preserved: if the model's actual context limit is exceeded,
the model API returns an error that the outer overflow-compaction retry
loop handles normally.
* fix(agents): assert non-null context engine in precheck skip log
* test(context-engine): cover owning precheck contract
* fix(agents): preserve precheck after context assembly failure
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Co-authored-by: Josh Lehman <josh@martian.engineering>
444a093593
