Add research tool for web-based research via scout agent

New tool that spawns a scout agent to perform web research and return
a structured research brief. The scout agent uses webdriver to browse
the web and returns a decision-ready report.

Changes:
- Added 'research' tool definition (12 core tools total)
- Added research tool dispatch in tool_dispatch.rs
- Created tools/research.rs implementation:
  - Spawns 'g3 --agent scout <query>' as subprocess
  - Captures stdout and extracts last line (report file path)
  - Reads and returns the report file contents
- Added exclude_research flag to ToolConfig
- Scout agent (agent_name == 'scout') does NOT have access to research
  tool to prevent infinite recursion
- Updated system prompts to describe when to use research tool
- Added scout.md agent prompt with research brief output contract

The research tool is preferred for complex research tasks (APIs, SDKs,
libraries, approaches, bugs). WebDriver can still be used directly for
simple lookups or fine-grained control.

crates/g3-core/src/tools/mod.rs

@@ -7,10 +7,12 @@
 //! - `todo` - TODO list management
 //! - `webdriver` - Browser automation via WebDriver
 //! - `misc` - Other tools (screenshots, code search, etc.)
+//! - `research` - Web research via scout agent
 
 pub mod executor;
 pub mod file_ops;
 pub mod misc;
+pub mod research;
 pub mod shell;
 pub mod todo;
 pub mod webdriver;