python-dev SKILL.md: add Pydantic/Protocol wisdom from lkml sprint

## Summary

The `python-dev` skill at `~/.claude/skills/python-dev/SKILL.md` needs updating:

1. **Fix mypy → ty** — SKILL.md line 14 still says `mypy --strict`, should be `ty`
2. **Add Pydantic construction patterns** — `create_model()` for dynamic model building
3. **Add Protocol/issubclass trap** — non-method members cause silent discovery failure
4. **Add plugin discovery patterns** — pkgutil + importlib auto-discovery
5. **Reference python-dev pack learnings** — `Packs/python-dev/learnings/`

## Learnings Source

Pack: `substrate/Packs/python-dev/` (commit 2d3af61)
- `pydantic-create-model-patterns.md`
- `protocol-issubclass-trap.md`
- `plugin-discovery-patterns.md`

## Work Items

- [ ] Fix mypy references → ty in SKILL.md
- [ ] Add "Wisdom" section referencing pack learnings
- [ ] Add use-when triggers for new patterns
- [ ] Compose full wisdom document from accumulated learnings (follow-up)

## Context

Source: lkml sprint WA2 (pydantic rewrite) and WA4 (plugin discovery)