CoS tip: drop oldest turns when compaction head exceeds context (#51121)

[kvnloo]

Origin

anomalyco#51121

Intent

When the compaction summarizer head exceeds the model's usable context (after reserving prompt/template budget), drop the oldest complete turns instead of giving up with "session too large to compact".

Proof (fail → pass)

cd packages/opencode
bun test test/session/compaction.test.ts -t "drops oldest turns when the head exceeds the model context budget"
# NEGATIVE (base product + tip tests): 0 pass / 1 fail
# GREEN: 1 pass / 0 fail
# restore: 0 pass / 1 fail

Notes

• Base: dev
• github_writes_origin=0
• Codex promote: parked