A non-converging file is skipped with a zero exit, so spotlessCheck reports success for a file it did not check #3143

What happens

For a file the formatter cannot converge on, Spotless prints

Skipping '<path>' because it does not converge.  Run {@code spotlessDiagnose} to understand why

and the check task passes — exit 0, BUILD SUCCESSFUL. The file is neither formatted nor reported as a violation,
so a green spotlessCheck is not evidence about that file at all.

Why it matters

In CI the file is silently ungated: the gate reports success while the file may be arbitrarily unformatted. A formatter
or plugin bump can also turn a previously-formatted file non-convergent with no signal — the gate goes from green to
green. It took a review of ours to notice a file was being skipped, and only because the message happened to be read.

Reproduction

Spotless 8.10.4 (Gradle plugin), format("markdown") with Prettier pinned at 3.9.6 — the trigger: the same file
converges under 3.9.10, so a reader on 3.9.10 sees no skip:

1. Add a markdown file whose fenced block is indented six spaces under a checklist item:

   - [ ] a task
   
         ```bash
         echo hi
         ```
   

2. ./gradlew spotlessMarkdownCheck --rerun-tasks prints the Skipping '<path>' because it does not converge. line
   above and exits 0.

With Prettier 3.9.10 the same file converges and no skip is printed; with 3.9.6 it does not. So the trigger in this case
is a version difference, but the silent skip is the gap either way — nothing about the message or the exit status
distinguishes "checked and formatted" from "not checked".

What would help

A way to make a skip fail the check — a failOnSkip-style option on the check task, or a non-zero exit when a target
file was not checked. A warning would help a human but not CI; the property worth having is that a green check means
every target file was actually checked.

Related, but a different ask: #1689 reports a convergence failure for ktlint — that the formatter converge on a
construct, not that a skip be visible.