CI: OS legs skip inert-only diffs, after a convention test for portable file names #73

Decided in decision 113, part of #118. The probe lane is gone (decision 112). The OS legs split into a windows lane and a macos lane. A selected run never has macOS: only the release PR into main, main, the nightly and the release run it. Windows is skipped when every changed path is prose (docs/**, design/**, .github/ISSUE_TEMPLATE/** or a *.md outside src/), and for run-time packages other than the Analyzers and Generators. test/, spikes/ and the agent tools keep Windows but skip the pack. PortableFileNameTests (conventions, on Linux) reads git ls-files. It refuses Windows' reserved characters and device names, a segment ending in a dot or a space, two paths equal under case folding and NFC normalization (directories too), and a path over 160 characters.

Problem

A PR that changes only docs/, design/ or *.md still runs the Windows and macOS legs (8.9 and 8.0 min). The Linux test leg already runs every convention and docs test. In the last 50 merged PRs this affected 5 PRs (10%). Their runs took about 17 minutes, where about 6 would do.

A docs-only PR can still break the Windows leg, though: for example, a file name Windows cannot check out (a reserved character, a trailing dot, two names that differ only in case).

Change

• A Linux convention test that refuses non-portable file names anywhere in the tree.
• After that, the selection from The Changes job fails open, misses renames, and does not list tools/Shared or .config/dotnet-tools.json #70 gains the lane os, selected unless every changed path is inert (docs/**, design/**, *.md). Anything else, including any path the list does not recognise, selects them.
• Push, release and nightly runs keep every leg.

Done when

The file-name test fails a seeded docs/a:b.md and a case collision, and a docs-only PR runs only Test (ubuntu-latest), Changes and the gate.