test(agents): the cases the review found

Each fixed finding gets a check that fails without its fix: inline code
and a list item's fence, a section removed from between two paragraphs,
a section before an unclosed fence, a CRLF file, no error when there is
nothing to write, a hard-linked and a read-only AGENTS.md, a dependency
whose project file sits in the consumer's own directory, a test's
dependency, and `test --list`. The read-only case needs an ordinary
user, since root writes through the permission bits, and is skipped
under root.

The merge cases go through a helper that counts a throw as a failure,
so one regression no longer ends the run before the cases after it. A
passing run removes its staged copy, whose builds leave a std PCM per
configuration in a tmpfs.

Reverting the inline-code rule, the own-notes exclusion, the skipping
of tests, the `test --list` guard or the paragraph-preserving removal
each fails its check.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/DependencyNotes/fixture/testonly/AGENTS.consumer.md

@@ -0,0 +1,3 @@
+## testonly
+
+Only a test uses this. It must never reach app.