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

Problem

A PR that changes only docs/, design/ or *.md still runs the Windows and macOS legs (8.9 and 8.0 min) and five probe legs (4.5 to 7.9 min each, mostly building the prober). 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 two lanes, os and probes, 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.