Remove final_output tool - let summaries stream naturally

- Remove final_output from tool definitions, dispatch, and misc tools
- Update system prompts to request summaries as regular markdown text
- Remove print_final_output from UiWriter trait and all implementations
- Remove final_output handling from agent core logic
- Rename final_output_summary → summary in session continuation
- Delete final_output test files
- Update tool count tests (12→11, 27→26)

This allows LLM summaries to stream through the markdown formatter
for a more natural, responsive user experience instead of buffering
everything into a tool call.

crates/g3-core/src/session_continuation.rs

@@ -32,8 +32,8 @@ pub struct SessionContinuation {
     pub created_at: String,
     /// Original session ID
     pub session_id: String,
-    /// The last final_output summary
-    pub final_output_summary: Option<String>,
+    /// Session summary (last assistant response)
+    pub summary: Option<String>,
     /// Path to the full session log (g3_session_*.json)
     pub session_log_path: String,
     /// Context window usage percentage when saved
@@ -50,7 +50,7 @@ impl SessionContinuation {
         is_agent_mode: bool,
         agent_name: Option<String>,
         session_id: String,
-        final_output_summary: Option<String>,
+        summary: Option<String>,
         session_log_path: String,
         context_percentage: f32,
         todo_snapshot: Option<String>,
@@ -62,7 +62,7 @@ impl SessionContinuation {
             agent_name,
             created_at: chrono::Utc::now().to_rfc3339(),
             session_id,
-            final_output_summary,
+            summary,
             session_log_path,
             context_percentage,
             todo_snapshot,