Improve release notes generator with MR grouping and links

Problem

The current release notes generator lists each commit separately, resulting in duplicate entries when an MR has multiple commits. It also lacks MR links, making it harder to trace changes.

Solution

Improve the release notes generator to:

1. Group commits by MR number (reduces 74 commits to 68 entries for 0.14)
2. Use MR titles instead of commit messages for better descriptions
3. Add HTML links to MRs in format <a href="URL">!XXX</a> (renders nicely when pasted to GitHub)
4. Parse conventional commit format from MR titles for proper categorization

Usage

bun run scripts/generate-release-notes.ts --since 0.13 --version 0.14.0

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Acceptance Criteria

• Release notes generator groups commits by MR number, reducing duplicate entries (e.g., 74 commits → 68 grouped entries for v0.14)
• MR titles are used as the primary description instead of individual commit messages
• HTML links to MRs are included in the output (format: <a href="URL">!XXX</a>) that render correctly when pasted to GitHub releases
• Conventional commit format is parsed from MR titles for proper categorization into feat/fix/chore/etc. sections

Definition of Done

• Generator updated and tested by regenerating v0.14 release notes
• Output shows grouped entries with MR links and proper categorization
• Orphan commits (without MR association) are still included as individual entries
• Both Markdown and JSON output modes work with the new grouping logic