ci: pin GitHub Actions to commit SHAs

Replace floating major-tag references (uses: foo/bar@vN) with the
specific commit SHAs they currently resolve to, annotated with the
fully-versioned tag (# vX.Y.Z) for human readability. Pinning to a SHA
makes the action behaviour reproducible across runs and removes the
supply-chain risk of a maintainer (or attacker) moving a tag to a new
commit.

One documented exception: semgrep/semgrep-action does not publish
v1.x.y semver tags — v1 is their only canonical release name on that
line — so it keeps a "# v1" annotation with an inline explanation.

actions/dependency-review-action's previous "@v5" reference would have
failed at run time: that repo only ships fully-versioned tags
(v5.0.0), no moving v5 alias. Pinned to v5.0.0 explicitly.

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

.github/workflows/security.yml

@@ -19,10 +19,10 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v6
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Set up Go
-        uses: actions/setup-go@v6
+        uses: actions/setup-go@4a3601121dd01d1626a1e23e37211e3254c1c06c # v6.4.0
         with:
           go-version-file: "go.mod"
 
@@ -48,7 +48,7 @@ jobs:
 
       - name: Upload vulnerability scan results
         if: failure()
-        uses: actions/upload-artifact@v7
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
         with:
           name: vulnerability-scan-results
           path: |
@@ -63,10 +63,10 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v6
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Set up Go
-        uses: actions/setup-go@v6
+        uses: actions/setup-go@4a3601121dd01d1626a1e23e37211e3254c1c06c # v6.4.0
         with:
           go-version-file: "go.mod"
 
@@ -84,7 +84,7 @@ jobs:
           echo "::endgroup::"
 
       - name: Run Semgrep security analysis
-        uses: semgrep/semgrep-action@v1
+        uses: semgrep/semgrep-action@713efdd345f3035192eaa63f56867b88e63e4e5d # v1 (no v1.x.y semver tag exists)
         with:
           config: >-
             p/security-audit
@@ -95,7 +95,7 @@ jobs:
 
       - name: Upload Semgrep SARIF results
         if: always()
-        uses: github/codeql-action/upload-sarif@v4
+        uses: github/codeql-action/upload-sarif@68bde559dea0fdcac2102bfdf6230c5f70eb485e # v4.35.4
         with:
           sarif_file: semgrep.sarif
         continue-on-error: true
@@ -110,22 +110,22 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v6
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Install libpcap
         run: sudo apt-get install -y libpcap-dev
 
       - name: Initialize CodeQL
-        uses: github/codeql-action/init@v4
+        uses: github/codeql-action/init@68bde559dea0fdcac2102bfdf6230c5f70eb485e # v4.35.4
         with:
           languages: go
           config-file: ./.github/codeql-config.yml
 
       - name: Autobuild
-        uses: github/codeql-action/autobuild@v4
+        uses: github/codeql-action/autobuild@68bde559dea0fdcac2102bfdf6230c5f70eb485e # v4.35.4
 
       - name: Perform CodeQL Analysis
-        uses: github/codeql-action/analyze@v4
+        uses: github/codeql-action/analyze@68bde559dea0fdcac2102bfdf6230c5f70eb485e # v4.35.4
         with:
           category: "/language:go"
 
@@ -138,10 +138,10 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v6
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Dependency Review
-        uses: actions/dependency-review-action@v5
+        uses: actions/dependency-review-action@a1d282b36b6f3519aa1f3fc636f609c47dddb294 # v5.0.0
         with:
           fail-on-severity: moderate
           allow-ghsas: GHSA-xxxx-xxxx-xxxx  # Add specific allowlisted advisories if needed