ci(docs): enumerate tracked files instead of walking the filesystem

[jsugg]

Problem

docs:validate walked the filesystem with a hardcoded skip list (.git,
coverage, node_modules, reports) and never consulted .gitignore. A
required check therefore returned a different answer depending on the
developer's disk:

docs:validate OK (12 Markdown files)   <- locally
docs:validate OK  (8 Markdown files)   <- CI, clean checkout

The extras were scratch notes under .local/, deliberately untracked.

Three consequences, in increasing order of how much they matter:

1. The gate's result is not reproducible — it depends on untracked local state.
2. It reads files that were deliberately kept out of version control.
3. Its failure mode is inverted: a local note saved with CRLF or containing
   a conflict marker fails docs:validate on the author's machine while CI
   passes, because CI never had the file. The gate fails the one person who
   cannot see why, and passes for everyone who can.

Change

Enumerate with git ls-files -z -- '*.md'. --root support is kept for tests.

The local set and the clean-checkout set are now identical by construction,
rather than by the skip list happening to keep pace with the directory layout —
which it had already stopped doing.

Acceptance

With .local/ present on disk:

$ npm run docs:validate
docs:validate OK (8 Markdown files)

$ diff <(git ls-files '*.md' | sort) <(docs:validate --json | jq -r '.files[]' | sort)
  (identical)

Note on the fixtures

They are now real git repositories. Writing a file to disk no longer makes it
visible to the gate — which is exactly the property being fixed, so it is
asserted directly rather than assumed: untracked Markdown is ignored, and
violations inside an untracked local note no longer fail the run. That last test
is the local-vs-CI inversion above, pinned so it cannot come back.

The drift this exposed, for the record: the audit measured 11 files, and it was
12 by the time the fix landed — a new untracked note had appeared in between.
The moving number is the argument.