feat: Externalize research tool as embedded skill

Replaces the built-in research/research_status tools with a portable
skill-based approach:

- Add embedded skills infrastructure (skills compiled into binary)
- Add repo-local skills/ directory support (highest priority)
- Create research skill with SKILL.md and g3-research shell script
- Script extraction to .g3/bin/ with version tracking
- Filesystem-based handoff via .g3/research/<id>/status.json
- Remove PendingResearchManager and all research tool code
- Update system prompt to reference skill instead of tool

Benefits:
- No special tool infrastructure needed (just shell + read_file)
- Context-efficient (reports stay on disk until needed)
- Crash-resilient (state persisted to filesystem)
- Portable (skill can be overridden per-workspace)

Breaking change: research tool calls now return a deprecation message
pointing to the research skill.

crates/g3-cli/src/ui_writer_impl.rs

@@ -820,13 +820,6 @@ impl UiWriter for ConsoleUiWriter {
             }
         }
 
-        // Add blank line before footer for research tool (its output is a full report)
-        if let Some(tool_name) = self.current_tool_name.lock().unwrap().as_ref() {
-            if tool_name == "research" {
-                println!();
-            }
-        }
-        
         // Check if we're in shell compact mode - append timing to the output line
         let is_shell = *self.is_shell_compact.lock().unwrap();
         if is_shell {