Add Markdown and shell indexing support

[findingnino]

Adds Markdown and shell script indexing.

• Indexes .md/.markdown files as Markdown, including file content, headings, and local links.
• Indexes .sh/.bash files as shell scripts, including file content, functions, and command calls.
• Adds extraction and source-file detection coverage.
• Fixes shell assignment parsing so empty assignments do not hang extraction.

Tested with Node 22:

• npm run build
• focused extraction tests for Markdown and shell support
• source-file detection tests