ci(pr-validation): shallow checkouts and scoped app.validate

The PR validation comment cloned TechAPI's full history twice (head and base)
and ran app.validate over every record. Check both out at depth 1, deepen the
head only to the merge base, fetch the pinned base at depth 1, and validate
just the changed records. integrity_check still scans both trees; it is
population-based and needs its own scoping.

Refs GetTechAPI/TechAPI#350

Author: Seungpyo1007

.github/workflows/techapi-pr-validation-comment.yml

@@ -73,22 +73,25 @@ jobs:
           repository: GetTechAPI/TechAPI
           ref: ${{ env.TECHAPI_HEAD_SHA }}
           path: TechAPI
-          fetch-depth: 0
+          # Shallow; deepened to the merge base in the next-but-one step.
+          fetch-depth: 1
 
       - name: Checkout TechAPI PR base
         uses: actions/checkout@v4
         with:
           repository: GetTechAPI/TechAPI
           ref: ${{ env.TECHAPI_BASE_SHA }}
           path: TechAPI-base
-          fetch-depth: 0
+          fetch-depth: 1
 
       - name: Pin PR base to the merge base
         shell: bash
         run: |
-          git -C TechAPI fetch --no-tags origin "$TECHAPI_BASE_SHA"
+          git -C TechAPI fetch --no-tags --depth=100 origin "$TECHAPI_BASE_SHA"
+          # Fall back to full history only if the PR branched from far back.
+          git -C TechAPI merge-base "$TECHAPI_BASE_SHA" HEAD >/dev/null 2>&1 ||             git -C TechAPI fetch --no-tags --unshallow origin
           base_sha="$(git -C TechAPI merge-base "$TECHAPI_BASE_SHA" HEAD)"
-          git -C TechAPI-base fetch --no-tags origin "$base_sha"
+          git -C TechAPI-base fetch --no-tags --depth=1 origin "$base_sha"
           git -C TechAPI-base checkout --detach "$base_sha"
           echo "TECHAPI_DIFF_BASE=$base_sha" >> "$GITHUB_ENV"
 
@@ -161,7 +164,7 @@ jobs:
           set +e
           {
             echo "## app.validate"
-            python -m app.validate
+            python -m app.validate --changed-since "$TECHAPI_DIFF_BASE"
             echo "app_validate_status=$?"
           } > validation.log 2>&1
           app_status=$(grep "app_validate_status=" validation.log | tail -n 1 | cut -d= -f2)