feat(tool): add experimental team tool for parallel subagents

[AndresCdo]

Issue for this PR

Closes #19999

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Introduces an experimental team tool for ephemeral parallel sub-agent orchestration.

MVP scope in this PR:

• Adds new team tool (packages/opencode/src/tool/team.ts) and usage guidance (team.txt).
• Supports executing multiple sub-agent tasks in parallel with bounded concurrency.
• Uses collect-all behavior: child failures do not cancel sibling tasks.
• Preserves child task_id values in aggregated output (prefers structured metadata, falls back to output parsing).
• Implements per-subagent permission preflight while preserving collect-all semantics when one subagent is denied.
• Adds config gate experimental.team_tool and registers tool only when enabled.
• Adds tests for:
  • tool execution + aggregation,
  • collect-all on child failure,
  • collect-all when one subagent permission is denied,
  • registry gating behind experimental.team_tool.

How did you verify your code works?

From packages/opencode:

• bun test test/tool/team.test.ts
• bun test test/tool/registry.test.ts
• bun typecheck

Additionally, push hook ran workspace typecheck successfully (bun turbo typecheck).

Screenshots / recordings

N/A (tool/runtime feature).

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR