Fix ChatGPT DOM extractor table handling.

Observed defect:
Tables in ChatGPT responses are being flattened into line-by-line text instead of Markdown tables.

Example bad output:
Goal

Good Plant Choices

Attract deer

Hostas, daylilies, many ornamentals

Feed pollinators

Native flowering perennials, shrubs, trees

Expected output:
| Goal | Good Plant Choices |
|---|---|
| Attract deer | Hostas, daylilies, many ornamentals |
| Feed pollinators | Native flowering perennials, shrubs, trees |
| Prevent deer damage | Deer-resistant species |
| Support overall biodiversity | A mix of native plants with different bloom times |

Requirements:
1. Update src/chatgptExtractor.js.

2. Add table-to-Markdown support:
   - Detect <table> elements.
   - Extract rows from <tr>.
   - Extract header cells from <th>.
   - Extract data cells from <td>.
   - Preserve cell order.
   - Trim cell text.
   - Convert inline formatting inside cells using existing inline extractor.

3. Markdown table rules:
   - First row should become the header row.
   - If the table has <thead>, use that as the header.
   - If no <thead> exists but the first row uses <th>, use that row as the header.
   - If no header exists, use the first row as the header for MVP.
   - Add separator row using `---`.
   - Escape pipe characters inside cells as `\|`.
   - Replace internal newlines in cells with `<br>` or a single space.

4. Example output format:
   | Goal | Good Plant Choices |
   |---|---|
   | Attract deer | Hostas, daylilies, many ornamentals |

5. Ensure table extraction is treated as a block element:
   - Tables should be separated from surrounding paragraphs by blank lines.
   - Do not also extract duplicate flattened table text from child nodes.

6. Preserve other existing behavior:
   - Paragraphs remain paragraphs.
   - Lists remain Markdown lists.
   - Code blocks remain fenced code.
   - Bold/italic/inline code remain inline.
   - Message order and role detection unchanged.

Suggested helper:
function tableToMarkdown(tableNode, context) {
  // return markdown table string
}

Acceptance criteria:
- ChatGPT tables export as valid Markdown tables.
- Headers and rows are preserved.
- Pipe characters inside cells are escaped.
- Tables are not duplicated as flattened text.
- Surrounding prose remains intact.
- No LLM calls.
- No Project Thoth application dependency.