SKILLS.md: document logging/formatting and type-hint conventions

[doomedraven]

Adds three conventions to SKILLS.md -> Coding Standards:

• Logging: pass args lazily (log.x("... %s", v)), never pre-formatted (f-string / % / .format() / +). Already enforced by the ruff G rules in pyproject.toml; documenting the reason (deferred formatting, stable message grouping).
• Non-logging strings: prefer f-strings.
• Type hints: project requires Python >= 3.10 -> builtin generics and X | None instead of typing.Dict/List/Optional, with the ruff command to convert touched files.

Docs only. Follow-up to #3263, where these were applied to modules/processing/network.py.