chore: .gitignore covers a symlinked node_modules, not only the directory form (#15763) (#15821)

`.gitignore:2` is `node_modules/` — the trailing slash makes it match a
DIRECTORY only. In a linked worktree whose `node_modules` is a symlink to a
shared install, the rule does not match: `git check-ignore -v node_modules`
exits 1 and `git status --short` prints `?? node_modules`.

Add the slashless form beside it (the existing line is untouched). This repo
tracks `node_modules` in no shape, so covering both forms costs nothing, and it
closes the `git add -A` hazard recorded twice before (#10781, #12583): an
untracked, non-ignored path at the repo root is one careless `git add -A` away
from committing a symlink to an absolute container path.

Measured effect it also removes: `dispatch-gates.mjs --changed` counts that
untracked path as part of the change set, and even derives a gate family from
it via `node_modules/@objectstack/spec/CHANGELOG.md`.


Claude-Session: https://claude.ai/code/session_012zGPuVVX3deAx9LdjK8jCk

Co-authored-by: Claude <noreply@anthropic.com>

Author: claude[bot]

.gitignore

@@ -1,5 +1,7 @@
 # Dependencies
 node_modules/
+# also without the slash: in a linked worktree node_modules can be a symlink, which is not a directory to git and so escapes the line above; the repo tracks node_modules in no shape (#15763)
+node_modules
 package-lock.json
 
 # Build output