Upgrade Plain Packages

1. Run the upgrade

uv run plain upgrade [package-names...]

This will show which packages were upgraded (e.g., plain-postgres: 0.1.0 -> 0.2.0).

2. Apply code changes for each upgraded package

For each package that was upgraded:

1. Run uv run plain changelog <package> --from <old-version> --to <new-version>
2. Read the "Upgrade instructions" section
3. If it says "No changes required", skip to next package
4. Apply any required code changes