feat(agent): add built-in research subagent

[Fengrru]

Issue for this PR

Closes #

Type of change

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

What does this PR do?

Adds a built-in read-only research subagent, following the same pattern as the existing explore subagent.

When given a task, it searches the web (websearch/webfetch) for tutorials and documentation, validates the local environment with read-only bash commands (which, --version), and returns a structured JSON report: steps, dependencies (with install/check commands and installed status), sources, and warnings. The intent is to invoke it before starting complex tasks that need external knowledge, so the main agent gets a validated plan instead of guessing.

Permissions are locked down like explore: "*": "deny" with only webfetch/websearch/bash/read/grep/glob allowed, and external directories read-only. It reuses the existing readonlyExternalDirectory and agent definition pattern, so no other code paths are touched.

Note: the docs mention a scout subagent for dependency source research; research does not overlap — scout inspects dependency repo source, research finds online tutorials/docs and checks the local environment.

How did you verify your code works?

The new subagent follows the exact structure of explore (same permission merge pattern, same prompt import mechanism), and the agent list sorts alphabetically so existing subagent-list test assertions are unaffected. Relying on this PR's CI (typecheck + test suites) to confirm.

Screenshots / recordings

Not a UI change.

Checklist

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

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