fix(ci): use PR titles instead of commit messages

[CalvinAllen]

Summary

Fix the changelog generation to use PR titles instead of commit messages/bodies.

Changes:

• Fetch PR number and title for each commit via GitHub API
• Skip commits that don't have an associated PR
• Track processed PRs to avoid duplicates
• Removes garbage entries caused by commit body parsing issues

Example Output

### 🐛 Bug Fixes

- #79 - fix(ci): restore conventional commit categorization

### ⚡ Performance & Improvements

- #73 - perf(ci): strip debug symbols from builds
- #75 - refactor(shim): implement lite ShimProvider interface

### 🎉 New Features

- #77 - feat(ci): enhance release notes with issue tracking

### 🔧 Maintenance

- #66 - chore(ci): add PR title linting
- #70 - docs: update documentation for today's changes

Test plan

• Run "Preview Changelog" workflow
• Verify no garbage/extraneous entries in output
• Verify entries show PR number and title

[CalvinAllen]

/skip-changelog