chore(maintainers): Document merge-batch workflow

Add a short maintainer guide for merge:batch and link it from the
existing merge policy docs.

Lock in the source-validation CI fixes discovered during the
merge-batch end-to-end exercise so shallow checkout and missing
base-branch fetch regressions fail the workflow contract test.

tools/scripts/tests/automation_workflows.test.js

@@ -133,11 +133,26 @@ assert.match(
   /- name: Audit npm dependencies[\s\S]*?run: npm audit --audit-level=high/,
   "CI should run npm audit at high severity",
 );
+assert.match(
+  ciWorkflow,
+  /source-validation:[\s\S]*?- uses: actions\/checkout@v4[\s\S]*?with:[\s\S]*?fetch-depth: 0/,
+  "source-validation should use an unshallowed checkout so base-branch diffs have a merge base",
+);
+assert.match(
+  ciWorkflow,
+  /source-validation:[\s\S]*?- name: Fetch base branch[\s\S]*?run: git fetch origin "\$\{\{ github\.base_ref \}\}"/,
+  "source-validation should fetch the PR base branch before changed-skill README credit checks",
+);
 assert.match(
   ciWorkflow,
   /- name: Verify README source credits for changed skills[\s\S]*?run: npm run check:readme-credits -- --base "origin\/\$\{\{ github\.base_ref \}\}" --head HEAD/,
   "PR CI should verify README source credits for changed skills",
 );
+assert.match(
+  ciWorkflow,
+  /source-validation:[\s\S]*?- name: Fetch base branch[\s\S]*?- name: Install npm dependencies[\s\S]*?- name: Verify README source credits for changed skills/,
+  "source-validation should fetch the base branch before running the changed-skill README credit check",
+);
 assert.match(
   ciWorkflow,
   /main-validation-and-sync:[\s\S]*?- name: Audit npm dependencies[\s\S]*?run: npm audit --audit-level=high/,