Add ty as non-blocking CI check

[filiplajszczak]

What is the problem or limitation you are having?

There is no type checking step in CI. Type errors are only caught during review or at runtime. See Type checking with ty (discussion) and tracking issue #2709.

Describe the solution you'd like

Add ty as a non-blocking tox environment so type check results are visible in CI without gating merges.

Specifically:

• Add [testenv:ty] to tox.ini with ignore_outcome = true
• Add [tool.ty.rules] section to pyproject.toml with rules set to warn
• Pin to ty==0.0.18
• Set unresolved-import = "ignore" (conditional imports like tomli, dmgbuild produce false positives)

Describe alternatives you've considered

• Run ty only locally: loses visibility for other contributors and reviewers
• Run ty as a blocking check: premature, too many diagnostics to gate on right now
• Skip ty entirely: misses the opportunity to surface type issues early

Additional context

ty checks all of src/briefcase/ in ~0.16s. Running non-blocking is the first step in the gradual introduction tracked in #2709.