chore: set up changesets for versioning and changelog generation

[joesobo]

Summary

• Normalize all package versions to 0.1.0 (nothing released yet)
• Install and configure @changesets/cli with independent versioning — each package versions on its own since plugins will eventually be extracted as standalone extensions
• Add seed CHANGELOG.md documenting initial 0.1.0 features
• Add changeset-bot GitHub Action to remind PR authors to include changesets
• Add changeset workflow guidance to CLAUDE.md (when to add, how to write good descriptions)

Test plan

• All 962 tests pass
• Typecheck clean across all 7 packages
• Lint clean
• pnpm changeset status runs without errors
• Verify changeset-bot comments on a future PR without a changeset

🤖 Generated with Claude Code

[changeset-bot]

⚠️ No Changeset found

Latest commit: 5887c59

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR