## Summary
Add persistence layer for todos so they survive across multiple turns in
a chat session. Todos are now saved to disk after each update and
automatically loaded and injected into the conversation context when a
new turn begins.

## Key Changes
- **New module `todo_persistence.ts`**: Provides utilities to save,
load, and delete todo JSON files stored in `.dyad/todos/<chatId>.json`
  - `getTodosFilePath()`: Constructs the file path for a chat's todos
- `saveTodos()`: Persists todos to disk with metadata (updatedAt
timestamp)
- `loadTodos()`: Loads previously saved todos, gracefully handling
missing or corrupted files
  - `deleteTodos()`: Removes the todos file when all todos are completed

- **Updated `local_agent_handler.ts`**: 
  - Load persisted todos at the start of each turn
- Emit loaded todos to the renderer immediately so the UI reflects them
- Initialize the agent context with persisted todos instead of an empty
array
- Inject a synthetic system message reminding the LLM of incomplete
todos from the previous turn, following the same pattern as existing
todo reminder logic
  - Ensure `.dyad/` directory is gitignored (idempotent operation)

- **Updated `update_todos.ts`**:
  - Persist todos to disk after each tool execution
  - Delete the todos file when all todos are marked as completed

## Implementation Details
- Todos are stored as JSON with an `updatedAt` timestamp for potential
future use
- File I/O errors are logged as warnings but don't interrupt the agent
flow (graceful degradation)
- The synthetic message injection for persisted todos only occurs if
there are incomplete todos, avoiding unnecessary context pollution
- The `.dyad/` gitignore step is idempotent and aligns with existing
patterns in the codebase

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---------
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Mohamed Aziz Mejri <mohamedazizmejri@Mohameds-Mac-mini.local>