🤖 fix: auto-clean completed sub-agents again (#2892)

Summary
- restore automatic cleanup for completed sub-agent workspaces instead
of retaining them in the sidebar indefinitely
- keep the patch-artifact safety gate so exec-style sub-agents still
finish artifact generation before deletion
- preserve the interrupted+reportedAt self-heal so stale completed rows
can still be recognized and pruned

Background
PR #2836 intentionally kept completed sub-agents around for optional
sidebar expansion. That was an undue UX consideration in practice: once
a sub-agent had finished, or had already produced a completed report
before later showing as `interrupted`, it could stick around forever
unless the user manually deleted it. Sub-agents used to disappear when
they were no longer in use, and this restores that behavior.

Implementation
- restore structural-leaf cleanup in
`TaskService.cleanupReportedLeafTask()` so completed descendants are
deleted again
- require ancestors to have no remaining child task nodes before they
can be auto-deleted, which avoids orphaning siblings while still
allowing cleanup to cascade upward once the last child is gone
- keep the restart/patch-artifact recheck path so exec and exec-derived
tasks still generate their patch artifacts before cleanup resumes
- update task-service regressions to cover immediate cleanup,
cleanup-after-patch, upward cascade, and interrupted+reportedAt deletion

Validation
- `bun test src/node/services/taskService.test.ts`
- `make static-check`

Risks
- Users lose the ability to browse completed sub-agent lineage from the
sidebar once cleanup runs. That is the intended tradeoff here: the prior
retention behavior regressed the default UX by leaving stale sub-agents
around indefinitely.

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Author: ammar-agent