feat(desktop): warn that deleting a parent keeps and archives its subtasks

[liuxiaocs7]

Summary

Deleting a parent task permanently removes it, but its ordinary linked subagent subtasks are intentionally kept and moved to the archive rather than cascade-deleted (#1467 / #3151 — each subtask owns its own transcript, artifacts, and outputs). The delete confirm never said so and the completion gave no feedback, so the archived rows that appeared next — labelled 原父任务已删除 on Settings › 活动 › 已归档任务 — read as tasks from nowhere.

This adds two notices, without touching deletion semantics:

• The delete confirm now appends how many subtasks will be kept and moved to Archived when the count is non-zero.
• The completion toast reports how many subtasks were moved.

The count is a renderer estimate off the live catalog in countArchivableSubtasks (app-shell-session-row-actions.ts), matching the Host's removal plan (session-retirement-coordinator.ts#readRemovalPlanSessionIds): ordinary subagents count, Agent Graph operators (subagentParent.graph) do not, already archived children do not, and revisions of one child count once (deduped by sessionRevisionFamilyId). Copy lives in shell-copy.ts (zh + en). A restore that calls the delete off shows no subtask notice, since nothing moved.

Fixes #3780

Verification

• npx biome lint on the four changed files — clean.
• tsc -p tsconfig.main.json --noEmit and tsc -p tsconfig.renderer.json --noEmit — both pass.
• Unit app-shell-session-row-actions-revisions.test.ts — 4/4 pass, including new cases: count excludes graph operators / archived / other parents and dedupes revisions; no notice when there are no subtasks; no notice when a concurrent restore calls the delete off.
• e2e parent-session-deletion.spec.ts — extended to assert the confirm names the subtask; not run in this environment (needs the Electron build + Playwright). Run with npm --workspace @maka/desktop run e2e -- parent-session-deletion.

Screenshots

Delete confirm — now warns the linked subtasks are kept and archived:

Completion toast — reports how many subtasks moved to Archived:

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Claude Code — traced the delete/archive path, wrote the copy, the countArchivableSubtasks helper, the wiring in deleteSession, and the unit/e2e coverage. Commit carries a Generated-by: Claude Code trailer.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No