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-cli/Cargo.toml

@@ -17,6 +17,7 @@ tracing = { workspace = true }
 tracing-subscriber = { workspace = true, features = ["env-filter"] }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
+serde_yaml = "0.9"
 rustyline = { version = "17.0.1", features = ["derive", "with-dirs"] }
 dirs = "5.0"
 tokio-util = "0.7"