Update the feature list format in K4000.

The current format in feature-list.md uses this structure:
```markdown
## F1: [x] Feature title
**Priority:** high | medium | low
**Complexity:** simple | moderate | complex
**Dependencies:** F2, F3 (if any)

Description of the feature...
```

This format has issues:
1. The checkbox `[x]` is inside the heading, which doesn't render as a proper checkbox in most markdown renderers
2. The metadata (Priority, Complexity, Dependencies) is verbose

Proposed new format:
```json
{
    "category": "functional",
    "description": "New chat button creates a fresh conversation",
    "steps": [
      "Navigate to main interface",
      "Click the 'New Chat' button",
      "Verify a new conversation is created",
      "Check that chat area shows welcome state",
      "Verify conversation appears in sidebar"
    ],
    "passes": false
  }
```

And the new output should we saved to feature-list.json

This task should:
1. Update the feature-list.json format specification in CLAUDE.md
2. Update the `save_features` tool to generate the new format
3. Update the `mark_feature_complete` tool to work with the new format
4. Ensure backwards compatibility or provide migration for existing feature lists
