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/streaming.rs

@@ -101,11 +101,15 @@ pub fn is_compact_tool(tool_name: &str) -> bool {
             | "coverage"
             | "rehydrate"
             | "code_search"
+            | "plan_approve"
     )
 }
 
 pub fn is_self_handled_tool(tool_name: &str) -> bool {
-    tool_name == "todo_read" || tool_name == "todo_write"
+    matches!(tool_name, 
+        "todo_read" | "todo_write" | 
+        "plan_read" | "plan_write"
+    )
 }
 
 /// Format a compact summary for a successful compact tool
@@ -125,6 +129,7 @@ fn format_compact_tool_summary(tool_name: &str, tool_result: &str) -> String {
         "coverage" => format_coverage_summary(tool_result),
         "rehydrate" => format_rehydrate_summary(tool_result),
         "code_search" => format_code_search_summary(tool_result),
+        "plan_approve" => format_plan_approve_summary(tool_result),
         _ => "✅ completed".to_string(),
     }
 }
@@ -512,6 +517,25 @@ pub fn format_code_search_summary(result: &str) -> String {
     }
 }
 
+/// Format a plan_approve result summary.
+pub fn format_plan_approve_summary(result: &str) -> String {
+    // Result formats:
+    // "✅ Plan approved at revision N. You may now begin implementation."
+    // "ℹ️ Plan already approved at revision N. Current revision: M"
+    // "❌ No plan exists to approve..."
+    if result.contains("❌") {
+        "❌ failed".to_string()
+    } else if result.contains("already approved") {
+        "ℹ️ already approved".to_string()
+    } else if let Some(pos) = result.find("revision ") {
+        let after = &result[pos + 9..];
+        let rev: String = after.chars().take_while(|c| c.is_ascii_digit()).collect();
+        format!("✅ approved rev {}", rev)
+    } else {
+        "✅ approved".to_string()
+    }
+}
+
 // =============================================================================
 // Tool Call Deduplication
 // =============================================================================