The fragment gate only ran validation when the changed path matched the
legal fragment name pattern, so `.changes/Foo.md`, `.changes/notes.txt`
and a symlinked fragment slipped through untouched and then broke the
next PR that added a legal fragment. The trigger now fires on any
top-level `.changes/` change other than README.md and rejects every
entry that is not README.md, released/, or a 100644 blob named
^[a-z0-9][a-z0-9._-]*\.md$.
The renderer had the same hole from the other side: `find -type f`
is false for symlinks, so a symlinked fragment was silently dropped
from the rendered notes, and the `[a-z0-9]*.md` glob only constrained
the first character so `chat reply.md` passed. It now walks every
top-level entry and fails on symlinks, unexpected directories,
non-regular files and illegal names. Both scripts pin LC_ALL=C so the
ASCII ranges cannot match uppercase under a different collation.
Adds regression coverage for illegal names, non-markdown entries,
symlinks and executable modes on both the gate and the renderer.
chichuan