feat: add compact UI output for Plan Mode tools

Plan tools (plan_read, plan_write) now display with elegant tree-style
formatting similar to the old todo_write UI:

- State indicators: □ (todo), ◐ (doing), ■ (done), ⊘ (blocked)
- Tree prefixes (├/└) for items with child details
- Strikethrough for completed items
- Shows touches and all three checks (happy/negative/boundary)
- Displays plan file path link at the end

plan_approve uses compact single-line format like read_file:
- Shows approval status and revision number
- Handles already-approved and error cases

Changes:
- Add print_plan_compact() to UiWriter trait with default impl
- Implement print_plan_compact() in ConsoleUiWriter
- Call print_plan_compact() from execute_plan_read/write
- Add plan_read/plan_write to is_self_handled_tool()
- Add plan_approve to is_compact_tool() with format_plan_approve_summary()
- Add serde_yaml dependency to g3-cli

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

@@ -758,16 +758,23 @@ pub async fn execute_plan_read<W: UiWriter>(
         None => return Ok("❌ No active session - plans are session-scoped.".to_string()),
     };
 
+    let plan_path = get_plan_path(session_id);
+    let plan_path_str = plan_path.to_string_lossy().to_string();
+
     match read_plan(session_id)? {
         Some(plan) => {
             let yaml = serde_yaml::to_string(&plan)?;
+            ctx.ui_writer.print_plan_compact(Some(&yaml), Some(&plan_path_str), false);
             Ok(format!(
                 "📋 {}\n\n```yaml\n{}```",
                 plan.status_summary(),
                 yaml
             ))
         }
-        None => Ok("📋 No plan exists for this session. Use plan_write to create one.".to_string()),
+        None => {
+            ctx.ui_writer.print_plan_compact(None, None, false);
+            Ok("📋 No plan exists for this session. Use plan_write to create one.".to_string())
+        }
     }
 }
 
@@ -826,6 +833,12 @@ pub async fn execute_plan_write<W: UiWriter>(
         return Ok(format!("❌ Failed to write plan: {}", e));
     }
 
+    // Display the plan in compact format
+    let plan_path = get_plan_path(session_id);
+    let plan_path_str = plan_path.to_string_lossy().to_string();
+    let yaml = serde_yaml::to_string(&plan)?;
+    ctx.ui_writer.print_plan_compact(Some(&yaml), Some(&plan_path_str), true);
+
     // Check if plan is now complete and trigger verification
     if plan.is_complete() && plan.is_approved() {
         let verification = plan_verify(&plan, ctx.working_dir);