feat(core): add task-parallel tool for concurrent subtasks - #60342

Summary

Adds a built-in task-parallel tool that fans out 2–5 independent subtasks concurrently, waits for all of them, and returns one combined summary.

This is a fresh V2 port of #47107 (the original PR was auto-closed by the stale-PR bot). dev has since moved the tool layer to packages/core/src/tool/plugin/, so the old branch could not be rebased; this implementation reuses the current subagent tool machinery.

How it works

• Validates 2–5 tasks and applies the same experimental.subagent_depth limit as subagent.
• Resolves each agent and asserts the task-parallel permission.
• Creates one child session per task, prompts it, starts a SubagentJob, then waits for every job with Effect.all (concurrency unbounded, max 5).
• Returns { results } plus a <parallel-tasks> summary.
• A failing task is reported as error; the others still complete.
• Backgrounded children are reported as running rather than silently dropped.

Tests

packages/core/test/tool-task-parallel.test.ts:

• runs two independent subtasks and returns one combined summary
• rejects fewer than 2 / more than 5 tasks
• reports a failed subtask without failing the others
• prevents nested fan-out via subagent_depth

bun test test/tool-task-parallel.test.ts → 5 pass.
bun run typecheck:tsc → clean.

Related

• [FEATURE]: add task-parallel tool to fan out independent subtasks concurrently #47105 — the feature request this implements
• feat(opencode): add task-parallel tool to fan out independent subtasks concurrently #47107 — original V1 PR (auto-closed by the stale-PR bot)
• [FEATURE]: add a built-in wait_for_subagents tool #58402 — complementary wait_for_subagents proposal