fix(agent-core-v2): keep user-cancellation identity when aborting swarm tool runs - #3729

Problem

Interrupting an AgentSwarm (>=2 items running) with Esc in the TUI killed the whole process with exit 1 within ~1s, taking the main agent and all subagents down mid-turn with nothing in the log. Seen twice in production on 0.42.0.

Two faults chain together:

1. human/agent/machine.ts aborted turn tools via entry.scope.abort() with no reason. The interruption therefore lost its UserCancellationError identity and surfaced as a generic DOMException("This operation was aborted"). In features/swarm/session/agentRunBatch.ts, the batchAbortListener's isUserCancellation check then failed, so the batch went down the fail() path and rejected instead of resolving per-subagent "aborted" results.
2. features/swarm/session/sessionSwarmService.ts attached cleanup via promise.finally(cleanup) and dropped the derived promise. When the batch rejects, that floating promise rejects unobserved -> unhandledRejection -> run-shell.ts emergencyExit(1).

Fault 2 stands on its own: any genuine AgentRunBatch failure (not just user interruption) crashed the process the same way.

Fix

• machine.ts: on the input.abort and abort-escalation paths, turn-tool scopes are now aborted with the user-cancellation reason (abortTurnToolsUserCancelled / stopTurnToolsUserCancelled / abortSpawnedTools), so an interrupted swarm batch resolves per-subagent "aborted" results as designed. The plain turn-exit path (abortTurnTools / stopTurnTools) still aborts without a reason, keeping the "turn over" semantics distinct from user cancellation. The first abort() latches the reason, so the later exit-path aborts cannot clobber it.
• sessionSwarmService.ts: the cleanup chain now ends in .catch(() => {}), so a rejecting batch can no longer crash the process through the floating derived promise. The returned promise still rejects to the caller unchanged.
• The human kernel cannot import _base (import-boundary rule: human is the pure kernel), so it gets its own UserCancellationError (userCancelled = true, name AbortError) in human/utils/abort.ts, and _base's isUserCancellation now also recognises cancellations structurally (userCancelled === true) across class copies.

Tests

• machine.test.ts: aborting a turn now passes a UserCancellationError reason to running turn tools.
• sessionSwarm.test.ts: a failing swarm batch no longer raises an unhandledRejection while run() still rejects to the caller.
• abort.test.ts: isUserCancellation recognises a foreign class copy flagged with userCancelled.

All three fail before the fix and pass after. Full agent-core-v2 suite (6466 tests), typecheck, lint:imports, oxlint, build, and the no-comments check are green.