[epic][js-changelog-tool] Build an AI tool to generate more meaningful changelogs

Right now, the changelog feels too granular because it’s generated from a raw diff. This makes it verbose and harder to read.

### High-Level Direction
Instead of building the changelog purely from a changelist, I think it should combine two sources:
- Emitter changelog – Higher-level changes like “Improved LRO API” rather than “Removed fooBegin”.
- TypeSpec spec (optional) – Domain-specific changes, e.g., “Added support for deleting a Compute Resource”.
### Challenges to Consider
- How do we pull data from the right places (spec + codegen)?
- How do we handle emitter versioning to pick the right changes?
- Specs aren’t versioned like packages, how do we deal with that?
### Example
Current format (too detailed):
```md
- Added operation AddonsOperations.createOrUpdate
- Added operation AddonsOperations.delete
...
```

Proposed direction (simpler, more readable):
```md
### Features Added
- Simplified long-running operations (LROs):
- Use a single method that supports both async and poller usage
- New helper for poller rehydration
- Paging improvements:
- Access continuation token from the page directly
```

/cc @lirenhe @joheredi