fix(ci): drop just from migration-assistant job (not preinstalled)

The 2a `migration-assistant` job ran `just install-grammar`, but
GitHub Actions ubuntu-latest runners do not ship the `just` task
runner preinstalled. Direct script invocation
`./editors/tree-sitter-rescript/scripts/install.sh` is equivalent
and removes the implicit dependency on a tool nothing else in this
workflow uses. The justfile recipe stays for local developer
ergonomics.

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

Author: hyperpolymath

.github/workflows/ci.yml

@@ -149,7 +149,12 @@ jobs:
         run: npm install -g tree-sitter-cli@^0.25.0
 
       - name: Build pinned tree-sitter-rescript grammar
-        run: just install-grammar
+        # Direct script invocation rather than `just install-grammar` —
+        # GitHub Actions runners do not ship `just` preinstalled, and
+        # there is no other recipe used in this workflow that justifies
+        # adding a setup step for it. The justfile recipe still exists
+        # for local developer ergonomics; both call the same script.
+        run: ./editors/tree-sitter-rescript/scripts/install.sh
 
       - name: Verify generated parser
         # `tree-sitter generate` is supposed to drop src/parser.c into