Consolidate task state by moving state.json into each task's folder and removing duplicated data.

## Current Problem

Data is duplicated between multiple files:
- `features` exists in both `state.json` and `feature-list.json`
- `questions` exists in both `state.json` and `questions.md`
- `status` exists in both `state.json` and `plan.md` frontmatter
- `name` and `project_path` are stored but derivable from folder structure

## Changes Required

### 1. Move state.json location
- From: `tasks/.state/my-task/state.json`
- To: `tasks/my-task/state.json`

### 2. Move review files location
- From: `tasks/.state/my-task/review-*.json`
- To: `tasks/my-task/review-*.json`

### 3. Slim down state.json to only non-duplicated data
Keep:
- `status`
- `created_at`
- `updated_at`
- `current_review`
- `review_iterations`
- `feature_approval_mode`

Remove:
- `name` (derivable from folder name)
- `project_path` (derivable from parent of tasks folder)
- `features` (already in `feature-list.json`)
- `questions` (already in `questions.md`)

### 4. Remove status from plan.md frontmatter
- Remove `status` field from `Frontmatter` struct
- Update `plan.md` generation to not include status

### 5. Delete .state directory
- After migration, the `tasks/.state/` directory is no longer needed

### 6. Update storage.rs
- Change path methods to point to new locations
- Update load/save methods to use new structure
- Remove methods that are no longer needed

### 7. Update code that reads features/questions
- Read features from `feature-list.json` instead of `state.json`
- Read questions from `questions.md` instead of `state.json`
- May need to parse `questions.md` to get question state

### 8. Migration path for existing tasks
- Provide migration for existing tasks in old structure
- Or handle both old and new locations during transition