docs(skills): add dataclass-over-NamedTuple guideline to usethis-python-functions skill

[Copilot]

Agents defaulted to NamedTuple for structured return types despite it leaking unintended tuple semantics (positional indexing, iteration, unpacking, positional equality). @dataclass is the correct default for grouping named fields.

Changes

• usethis-python-functions skill (v1.3 → v1.4): adds a new "Prefer dataclasses over NamedTuples for structured types" section covering:
  • Why NamedTuple is the wrong default (tuple semantics leak implementation details)
  • When NamedTuple is genuinely appropriate (positional comparison, iteration, interop with tuple-expecting code)
  • Canonical example contrasting the two approaches
  • Common mistakes (choosing NamedTuple for brevity or speculative future unpacking)

# Bad: positional indexing silently works; fragile on field reorder
class Result(NamedTuple):
    stdout: str
    stderr: str

result[0]  # compiles and runs; wrong if fields ever reorder

# Good: only named access available
@dataclass
class Result:
    stdout: str
    stderr: str

result.stdout  # explicit and refactor-safe

Placed in usethis-python-functions alongside the existing "Avoid returning tuples" section, which already recommends dataclasses as the structured alternative.