fix(session): cancel subtask child sessions

[kitlangton]

Summary

• Propagate Task tool cancellation to child sessions when parent slash-command subtasks are interrupted.
• Make TaskPromptOps cancellation awaitable so release-time cleanup can wait for child session cancellation.
• Add regression coverage for slash-command subtask cancellation and direct Task tool abort signals.

Regression Origin

Likely introduced by c5fb628 (refactor(tool): Tool.Def.execute returns Effect, rename defineEffect → define (#21961)) on 2026-04-10, when slash-command subtask execution stopped passing the Effect-provided interrupt signal directly into TaskTool and started using a separate AbortController.

Verification

• bunx prettier --check packages/opencode/src/tool/task.ts packages/opencode/src/session/prompt.ts packages/opencode/test/tool/task.test.ts packages/opencode/test/session/prompt.test.ts
• bunx oxlint packages/opencode/src/tool/task.ts packages/opencode/src/session/prompt.ts packages/opencode/test/tool/task.test.ts packages/opencode/test/session/prompt.test.ts (warnings only, existing rules)
• bun typecheck from packages/opencode
• bun run test -- test/tool/task.test.ts
• bun run test -- test/session/prompt.test.ts -t "cancel finalizes subtask tool state|cancel propagates from slash command subtask to child session|running subtask preserves metadata|running task tool preserves metadata"
• pre-push bun turbo typecheck

[Hona]

nice yea i ran into this before