Closes #110572

What Problem This Solves

Fixes an issue where a parent agent that yielded after receiving a child completion could remain stalled instead of resuming to send its final answer. The failure was timing-dependent and could also occur while completion delivery was still in progress.

Why This Change Was Made

The registry now records the exact requester attempt for completion-producing children, persists explicit yield intent before aborting that attempt, and retains the child rows until requester settlement. Yielded batches are durably re-armed with generation-guarded wake state so stale delivery work cannot erase a newer wake. Only completions already delivered or in progress when the requester yields request a fresh turn; children that finish later keep the normal single-delivery path, avoiding duplicate wakes. State remains in the existing SQLite payload and does not require a schema-version change.

This is the owner-boundary fix: requester-attempt settlement owns the race, while child delivery and cleanup keep their existing responsibilities. The regression was introduced by #99396; that guard was intentional, but missed the active-requester yield window.

Implementation and review were AI-assisted.

User Impact

Parent agents now reliably resume and deliver their final response after child work finishes, including the narrow race where completion delivery overlaps sessions_yield. Future child completions still produce one wake, not a duplicate.

Release note: Fix parent-agent handoffs stalling after an already-delivered child result when the parent yields.

Evidence

• Exact-head Blacksmith Testbox: tbx_01kxvat3rnrwnek1se2smzna41, succeeded in 244.252 seconds.
• 157 focused tests passed across requester settlement, registry lifecycle, persistence, SQLite round-trip, attempt settlement, and sessions_yield.
• Core and core-test tsgo checks passed.
• Targeted formatting and lint passed for all 19 touched files.
• Max-lines ratchet passed.
• QA subagent-handoff scenario passed 1 of 1 with mock OpenAI transport on the exact branch head.
• Full changed-surface gate passed earlier in the same branch series: formatting, plugin boundaries, core and core-test types, full core lint shards, database guard, and cycle checks.
• Fresh branch autoreview reported no accepted or actionable findings.