Migrate research and research_status tools from core tools to a
dynamically loadable toolset, following the same pattern as webdriver.
Changes:
- Add 'research' toolset to TOOLSET_REGISTRY in toolsets.rs
- Add create_research_tools() function with research and research_status
- Remove research tools from create_core_tools() in tool_definitions.rs
- Remove exclude_research field and with_research_excluded() from ToolConfig
- Update tests: core tools now 13 (was 15), added 3 research toolset tests
The agent must now call load_toolset('research') to use research tools.
This simplifies the default tool set and removes special-case logic for
the scout agent (which simply won't load the research toolset).
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