refactor(core): share subagent completion delivery - #46054

Why

Live subagent completion and restart recovery independently build the same parent notification, admit it to the Session, and acknowledge the background record. Changes to the message or delivery ordering therefore need to remain synchronized across two implementations.

This is a small behavior-preserving consolidation, not a notification-lifecycle redesign.

What Changes

Both callers use SubagentCompletion.deliver for result formatting, synthetic admission, and acknowledgment. The operation receives only the Session and Job capabilities it needs; it introduces no service, registry, or owned lifetime.

flowchart LR
    Live[Live subagent completion] --> Deliver[SubagentCompletion.deliver]
    Restart[Recovered subagent result] --> Deliver
    Deliver --> Admit[Session.synthetic]
    Admit --> Ack[Job.completeBackground]

Live and recovery callers keep their different preflight checks, assistant-text selection, and missing-session policies. Production changes total +55/-59, including the new 32-line module: four net lines deleted, with one shared delivery implementation.

Scope

Subagent delivery only. Shell completion, manual-shell scheduling, Job persistence, and public contracts are unchanged. Observer ownership remains plugin-scoped, so this does not fix completion loss during plugin replacement or change the direct-SDK lifetime work in #45988.

Verification

# packages/core
bun run test test/job.test.ts test/session-execution.test.ts test/tool-subagent.test.ts
bun typecheck

# repository worktree
bunx prettier --check packages/core/src/session/subagent-completion.ts packages/core/src/session/execution/restart.ts packages/core/src/tool/plugin/subagent.ts packages/core/test/session-execution.test.ts packages/core/test/tool-subagent.test.ts
git diff --check

• All 57 focused tests pass, with 324 assertions. Core typecheck and formatting pass.
• The TestLLM-backed regression exercises live completion and then replays its persisted background marker, verifying one durable notification and acknowledgment after admission. It passed five additional repetitions during implementation.
• A real synthetic-admission identity conflict verifies that failed delivery retains the marker.
• No full Core suite, live-service testing, plugin-reload lifetime test, or integration run against feat(sdk): add scoped direct Session handles #45988 was performed. Missing-session policies and other unchanged branches were source-compared and retain existing coverage.