ci: pin all GitHub Actions to full SHA digests

Pin every third-party GitHub Action to its current commit SHA with a
version comment, eliminating supply chain risk from mutable version
tags. Mutable tags (v4, v2, etc.) can be force-pushed by upstream
maintainers; SHA digests are immutable.

18 unique actions pinned across 9 workflow files.

Closes #357

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/workflow-sanity.yml

@@ -26,7 +26,7 @@ jobs:
     timeout-minutes: 10
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
 
       - name: Fail on tabs in workflow files
         shell: bash
@@ -59,7 +59,7 @@ jobs:
     timeout-minutes: 10
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
 
       - name: Lint GitHub workflows
-        uses: rhysd/actionlint@v1.7.11
+        uses: rhysd/actionlint@393031adb9afb225ee52ae2ccd7a5af5525e03e8 # v1.7.11