[WIP] feat: subagents

[uinstinct]

Description

This PR introduces isolated sub agents to work on specialized tasks.

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Checklist

• [] I've read the contributing guide
• [] The relevant docs, if any, have been updated or created
• [] The relevant tests, if any, have been updated or created

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Tests

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Summary by cubic

Introduce subagents to the CLI. You can now spawn a specialized agent via the Subagent tool to handle multi‑step tasks autonomously with live streaming output.

• New Features

  • Added Subagent tool with parameters: description, prompt, subagent_name.
  • Built-in “general” agent with its own system prompt.
  • Executor runs subagents in a child session, isolates system message, and temporarily disables chat history interference.
  • Allows all tools inside subagents for now.
  • Streams tool call output using toolCallId; UI shows live subagent output and truncates long logs.
  • Added task metadata footer with status.
  • Subagent tool listed among built-ins and always available.

• Migration

  • Tool.run now supports an optional second parameter: context?: { toolCallId: string }.
    • If your TypeScript settings are strict, update custom tools to match the new signature:
      • Before: run(args) => Promise
      • After: run(args, context?) => Promise

^{Written for commit 2003128. Summary will update automatically on new commits.}

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📚 Documentation PR created!

I've reviewed the changes in this PR and created PR #9129 with comprehensive documentation for the Subagent feature.

The documentation includes:

• Architecture overview and execution flow
• Built-in agents (general agent)
• When to use subagents vs. direct tool calls
• Tool interface, parameters, and examples
• Output streaming behavior
• Implementation details and migration notes for custom tools
• Error handling
• Future enhancements

The docs are added to extensions/cli/docs/subagents.md and extensions/cli/AGENTS.md is updated to reference them, following the same structure and level of detail as existing CLI documentation like artifact-uploads.md and storage-sync.md.

Please review when you have a chance!

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📚 Documentation PR #9129 is ready for review

I've created comprehensive documentation for the Subagent feature. The PR is experiencing some transient CI failures (sqlite download 503 errors) that are unrelated to the documentation changes - all code quality checks (prettier, lint, core-checks, etc.) are passing.

The documentation covers:

• Architecture overview and execution flow
• Built-in agents (general agent)
• When to use subagents vs. direct tool calls
• Tool interface, parameters, and examples
• Output streaming behavior
• Implementation details and migration notes for custom tools
• Error handling
• Future enhancements

Feel free to review the docs while the CI infrastructure issues resolve themselves!

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