feat(core): add scout agent for repo research

[nexxeln]

summary

• add a built-in scout subagent for external docs and dependency-source research
• add managed repo_clone and repo_overview tools with shared repository parsing and cache paths under app data
• document scout across the built-in agent docs, including localized agents.mdx pages

testing

• bun test test/agent/agent.test.ts test/tool/repo_clone.test.ts test/tool/repo_overview.test.ts
• bun typecheck

[nexxeln]

/review

[github-actions]

lgtm