feat: async research tool - runs in background, returns immediately

The research tool now spawns the scout agent in a background tokio task
and returns immediately with a research_id placeholder. This allows the
agent to continue working while research runs (30-120 seconds).

Key changes:
- New PendingResearchManager for tracking async research tasks
- research tool returns immediately with placeholder containing research_id
- research_status tool to check progress of pending research
- Auto-injection of completed research at natural break points:
  - Start of each tool iteration (before LLM call)
  - Before prompting user in interactive mode
- /research CLI command to list all research tasks
- Updated system prompt to explain async behavior

The agent can:
- Continue with other work while research runs
- Check status with research_status tool
- Yield turn to user if results are critical before continuing

crates/g3-core/src/tool_dispatch.rs

@@ -43,6 +43,7 @@ pub async fn dispatch_tool<W: UiWriter>(
 
         // Research tool
         "research" => research::execute_research(tool_call, ctx).await,
+        "research_status" => research::execute_research_status(tool_call, ctx).await,
 
         // Workspace memory tools
         "remember" => memory::execute_remember(tool_call, ctx).await,