Get full details of a card by its number or ID.
Retrieve complete card data including description, steps count, and metadata.
**When to use:**
- Need full description or metadata for a specific card
- Check step completion status or see all tags/assignees
**Don't use when:** Scanning multiple cards - use `fizzy_search` first.
**Arguments:**
- `account_slug` (optional): Uses session default if omitted
- `card_number` (recommended): The human-readable `#` number from URLs/lists (e.g., 42)
- `card_id` (alternative): The UUID from API responses. Use `card_number` when possible.
**IMPORTANT:** Provide `card_number` (integer) OR `card_id` (string UUID), not both.
The `card_number` is the `#` visible in the UI (e.g., #42). The `card_id` is the internal UUID.
**Returns:**
JSON with id, number, title, description (markdown), status, board_id, column_id, tags array, assignees array, steps_count, completed_steps_count, comments_count, url, created_at, updated_at, closed_at (null if open).
Example: `{"id": "card_abc", "number": 42, "title": "Fix bug", "status": "open", "steps_count": 3, ...}`
**Related:** Use `fizzy_comment` or `fizzy_step` for deeper interaction.