feat(cli): parallel subagents

[uinstinct]

Description

add parallel subagents

requires #10830 to merged first

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• [] 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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Summary by cubic

Enable parallel subagent execution in the CLI with live output. The subagent tool is now enabled by default and adds built-in Navigator and Code Reviewer agents.

• New Features

  • SubAgentService to run subagents with streaming output and event hooks; prevents nested calls.
  • Built-in Navigator and Code Reviewer subagents for Anthropic models with direct API keys.
  • Dedicated SubAgentOutput UI to display live subagent logs in TUI.
  • Improved subagent tool description and made the tool always available.

• Refactors

  • Replaced the executor with SubAgentService and registered it in the service container.
  • Merged config and built-in subagents in get-agents and created LLM APIs per subagent.
  • Removed the subagent beta flag and CLI option; simplified toolsConfig.
  • When ChatHistoryService is disabled, handleToolCalls now writes tool results to chat history.

^{Written for commit fe31959. Summary will update on new commits.}