[atomantic]

Summary

Third slice of #16011. It retires the last multi-generation post-absence settlement sub-machine: settling a predecessor once a successor proves it delivered the same head.

Done ✓

• A completed cleanup run whose task has moved to another owner now settles its own physical cleanup directly, once all of these hold:
  • its checkout path(s), local branch and worktree entry are gone;
  • no live run owns the branch.
• Its execution result is unchanged. The ledger records already-absent-after-handoff, with no integration claim, so the agent card shows the worktree as cleaned up without saying the code merged.
• The writer settles a captured checkout only once the move is final: the task has completed, or it names another cleanup owner. None of the ownership writers can hand either state back to the run. An unowned task, the run's own pending handoff, or an unreadable task stays pending, because agentFinalization writes a run's cleanup handoff after the run is marked complete.
• The periodic completion sweep now settles these runs too. Before, only a matching checkout:retired receipt could, through settleSeededPredecessor.
• Removed:
  • settleSeededPredecessor;
  • its 25-hop explicitSameHeadResume chain walk;
  • the moved-predecessor matching and newest-first ordering in the checkout:retired wake;
  • recordSuccessorCheckoutRetirement and the successor branch of the retirement writer;
  • inspectAbsentCheckout in heldCheckoutRecovery.js, whose only caller was settleSeededPredecessor.
• settleAbsentSeedRetirement and the new handed-off settlement share one absence/owner check, settleAbsentScope.
• Compatibility: archived records the old successor writer already settled are terminal (cleanup and verification both completed). Nothing re-selects them, so no migration is needed.

Remaining ▢ (tracked on #16011)

• readAbsentTaskSnapshot / settleAbsentTaskCompletion, the removed-task replay that reads the compressed completion snapshot, can fold into the same direct-absence settlement. This does not depend on Retire two-step digest-bound held-checkout review ceremony and REVIEW_RETIREMENT sub-agent #16016.
• captureCheckoutSeeds / filesystemIdentity / readCompletedCheckoutIdentity incarnation capture and sameTaskRetirementBinding are still used by:
  • branchCompletionPolicy, heldCheckoutRecovery, checkoutDeliveryContinuation and resumedCheckoutDelivery, which the in-flight Retire two-step digest-bound held-checkout review ceremony and REVIEW_RETIREMENT sub-agent #16016 is retiring;
  • release-publication lineage.
    They should follow Retire two-step digest-bound held-checkout review ceremony and REVIEW_RETIREMENT sub-agent #16016.

Found while tracing ownership: #16037. The pre-existing own-seed absence settlement can strand a cleanup-held task whose owner's capture never completed. It predates #16032 and is filed separately; this PR does not change that path.

Local review

The configured required reviewer (gpt-6.1-terra, ~max=2) ran 2 rounds.

• Round 1 (cf93291) raised one finding, accepted and fixed in c6dcb28. The finding: a bare cleanup-owner-changed verdict can be transient right after completion, because the run's own cleanup handoff is written to the task after completeAgent. Settling in that window could leave the task without a delivery verifier. The fix: the writer now settles a captured checkout only once the task completed or names another cleanup owner. The writer test pins this.
• Round 2 (c6dcb28, the published head) was clean, with no findings.

Test plan

• agentRepoStateVerification: a new case covers settlement through both the sweep and the receipt wake. It also covers each hold: the run still owns cleanup, a live owner, the checkout present in either path spelling, the local ref present, a registered worktree. It fails on the pre-change source.
• cosAgentLifecycle: a new writer contract case covers a completed task, another named owner, a pending handoff, the run's own handoff, a missing task, a CAS refusal, the wrong path and a non-cleanup run. Its three refusal states fail without the ownership guard.
• Green: the agentRepoStateVerification, cosAgentLifecycle, heldCheckoutRecovery and completedCheckoutEvidence suites, plus 19 adjacent completion, retirement and held-checkout suites (653 tests).
• npm run pregate passed.

Refs #16011