Move away from ruamel.yaml and pyyaml somehow

ruamel.yaml is used by semiwrap update-yaml command specifically because it can read a YAML file and write it while retaining comments. Unfortunately, it's not pure python, and the creator decided to no longer publish wheels for the C portion starting with release 0.19. This is annoying for a bunch of reasons, so I'd like to move away from it.

Possibilities:

• Switch to some other configuration language
  • I tried TOML and hated it
• Find a pure python YAML library that can round-trip comments
  • strictyaml claims to do this, but it has its own weird quirks. Would need to look harder at it sometime
• Alternatively, maybe we could just vibe-code a pure python implementation of the C library using the existing test suite as a starting point? (prior art: https://simonwillison.net/2025/Dec/15/porting-justhtml/)
  • ruamel.yaml has a pure python implementation, forking that so that it's used by default might be a way to go?

I've pinned to 0.18 for now, but once Python 3.15 comes out then we'll need to have figured something out.

Start at the semiwrap update-yaml command and inspect how ruamel.yaml currently reads and writes configuration. Review the existing test suite as the starting point for evaluating alternatives. Done means the command can replace the pinned dependency while retaining YAML comments and supporting the project’s stated Python-version needs.