Add Plan Mode to replace TODO system

Plan Mode is a cognitive forcing system that requires reasoning about:
- Happy path
- Negative case
- Boundary condition

New tools:
- plan_read: Read current plan for session
- plan_write: Create/update plan with YAML content (validates structure)
- plan_approve: Mark current revision as approved

New command:
- /feature <description>: Start Plan Mode for a new feature

Plan schema requires:
- plan_id, revision, approved_revision
- items with id, description, state, touches, checks (happy/negative/boundary)
- evidence and notes required when marking items done

Verification:
- plan_verify() called automatically when all items are done/blocked

Removed:
- todo_read, todo_write tools
- todo.rs module and related tests

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

@@ -4,7 +4,7 @@
 //! Tools are organized by category:
 //! - `shell` - Shell command execution and background processes
 //! - `file_ops` - File reading, writing, and editing
-//! - `todo` - TODO list management
+//! - `plan` - Plan Mode for structured task planning
 //! - `webdriver` - Browser automation via WebDriver
 //! - `misc` - Other tools (screenshots, code search, etc.)
 //! - `research` - Web research via scout agent
@@ -16,9 +16,9 @@ pub mod acd;
 pub mod file_ops;
 pub mod memory;
 pub mod misc;
+pub mod plan;
 pub mod research;
 pub mod shell;
-pub mod todo;
 pub mod webdriver;
 
 pub use executor::ToolExecutor;