fix(lint): add vendored-skill dirs to markdownlint ignores; drop dead patterns

The lint walk fix in commit 115196f28 dropped `.agents/skills/` and
`.claude/skills/` from the python / shellcheck / generic walks but did
NOT extend the same exclusion to `markdownlint-cli2`, which has its
own glob config in `.markdownlint-cli2.jsonc`. As a result every
`pre-commit run make-lint-markdown` flagged 14 baseline findings in
vendored skill README / SKILL.md files (MD041 first-line-heading,
MD045 no-alt-text) — those ship from upstream skill repos and follow
their own conventions.

Add `.agents/skills/**` and `.claude/skills/**` to the `ignores` array.
Drop the two patterns that match zero files today:

- `tests/**/fixtures/**` — there is no `fixtures/` directory anywhere
  under `tests/`; the actual fixture files are Python modules named
  `_fixtures.py` (already covered by the markdownlint glob's `**/*.md`
  restriction).
- `docs/changelog/archive/**` — there is no `archive/` directory under
  `docs/changelog/`.

`.github/PULL_REQUEST_TEMPLATE/**` is kept (still matches the six
template files).

Verified: `make lint-markdown` now reports `0 error(s)` over
686 files (was 716 walked / 14 errors before).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.markdownlint-cli2.jsonc

@@ -33,8 +33,8 @@
     "venv/**",
     ".venv/**",
     "**/__pycache__/**",
-    "tests/**/fixtures/**",
     ".github/PULL_REQUEST_TEMPLATE/**",
-    "docs/changelog/archive/**"
+    ".agents/skills/**",
+    ".claude/skills/**"
   ]
 }