feat(deps): remove blackfriday markdown parser dependency

[perber]

No description provided.

[Copilot]

Pull request overview

This PR removes the blackfriday Markdown parser dependency by switching Markdown-to-text extraction and heading parsing to goldmark, aligning search indexing and excerpt generation with the already-present goldmark dependency in the codebase.

Changes:

• Replace blackfriday-based heading extraction in the SQLite search indexer with a goldmark AST walk.
• Replace blackfriday.Run in excerpt.PlainTextFromMarkdown with goldmark.Convert + sanitization.
• Remove blackfriday (and now-unused fsnotify) from go.mod/go.sum, and add unit tests covering heading extraction behavior.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/search/sqlite_index.go	Migrates heading extraction from Blackfriday to Goldmark AST parsing for search indexing.
internal/search/sqlite_index_test.go	Adds focused tests for extractHeadings to validate expected heading-only extraction and formatting stripping.
internal/core/excerpt/excerpt.go	Migrates Markdown-to-plain-text conversion from Blackfriday to Goldmark while keeping HTML sanitization.
go.mod	Removes Blackfriday (and unused fsnotify) dependencies; retains Goldmark.
go.sum	Cleans up checksums corresponding to removed dependencies.

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