feat(compaction): fold to a token-capped tail, not recent turns

[TheGreatAxios]

Summary

• Folds keep a token-capped tail instead of the last N turns verbatim.
• Pair atomicity still holds: a tool call and its result stay together or both excerpt.
• Fat handoff file remains the durable working state; the live prompt is the thin spine plus that tail.

Verification

• pre-commit typecheck + build passed on commit
• Targeted compaction suite was not finished on this branch (work was interrupted). Treat CI as the first full gate.

Part of CL-9489

[linear-code]

CL-9489

[TheGreatAxios]

Review

Incomplete, not shippable. The verbatim last-N floor is gone and the CL-9006 latch is not weakened, but the live fold is still spine + unexcerpted anchors + tail, the tail cap has holes, still-over folds are silent, and the pairing suite is red.

Blocking

1. Unexcerpted anchors blow the 60% trigger. apply still emits [summaryTurn, ...anchorTurns, ...tailTurns]. edit_file/write_file score 10 and drag full bodies (excerpting is tail-only). A fixture of 8 fat edit pairs landed at ~160k tokens vs a 76.8k trigger.

2. Non-convergence is not reported. foldNonConverged is a getter only; the director never reads it. After a still-over fold the consecutive cap is spent and the session sits over 60% with no operator notice.

3. Pairing suite is red (compactor-pairing.test.ts, 8 fails). When selectTail keeps every turn, the early summarizedTurns.length === 0 return skips stubSupersededReads.

4. Images are free against the tail cap. excerptedTurnChars ignores image blocks, so a huge newest screenshot can keep tailTokenEstimate ≤ 2500 while occupancy stays over 60%.

Should-fix

preserveWholeUserMessages is declared and never read by selectTail.

What holds

• COMPACTOR_KEEP_RECENT_TURNS / keepRecentTurns deleted; default tail budget 2500 tokens.
• Pair atomicity for well-formed call/result sequences still holds.
• Fat handoff blob + thin spine pointer remain.

[TheGreatAxios]

Review (second pass, 25ab7a67)

Shippable. All four prior blockers are closed. Independent tester: 172 pass / 0 fail; typecheck exit 0. CL-9006 latch is not weakened.

1. Live fold is spine + token-capped tail. Fat edit_file/write_file bodies go to the handoff; occupancy fixture < 76800.
2. Still-over folds emit custom.compaction.fold_nonconverged (TUI systemNotice / exec warn).
3. stubSupersededReads still runs when the tail swallows the transcript. Pairing suite green.
4. Images count via estimateContentBlockTokens.

Hygiene for later (not blocking): dead-ish preserveWholeUserMessages / maxAnchorTurns leftovers; screenshot budget test can be made stricter.