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

Cargo.lock

@@ -1384,6 +1384,7 @@ dependencies = [
  "rustyline",
  "serde",
  "serde_json",
+ "serde_yaml",
  "sha2",
  "syntect",
  "tempfile",