fix(scrape): emit markdown table separators

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53179.

html_to_markdown() now renders HTML tables through a table-specific path that collects rows/cells and inserts a GitHub-flavored Markdown separator row after the first row. Cell content still goes through the existing markdown conversion path, then is flattened and pipe-escaped for table cells.

Validation:

• cargo +1.90.0 fmt --package cortex-cli --check
• isolated rustc +1.90.0 --test harness covering header separator output
• git diff --check

Note: cargo +1.90.0 test -p cortex-cli test_html_to_markdown_table_includes_separator -- --nocapture is blocked on this Windows machine by the existing workspace aws-lc-sys MSVC build failure before cortex-cli tests run.

[greptile-apps]

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