[codex] Pin GitHub Actions workflow references (#57)

## Summary
Pin floating external GitHub Actions workflow refs to immutable SHAs.

## Why
See the rationale doc:
https://docs.google.com/document/d/1qOURCNx2zszQ0uWx7Fj5ERu4jpiYjxLVWBWgKa2wTsA/edit?tab=t.0

## Validation
- `rg -n --pcre2
"uses:\s*(?!\./)(?!docker://)[^#\n]+@(?![0-9a-f]{40}(?:\s+#.*)?$)\S+"
.github/workflows`
- `git diff --check`
- `git diff --stat -- .github/workflows`

Author: hintz-openai

.github/workflows/make-all.yml

@@ -15,17 +15,17 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
 
       - name: Set up mise tools
-        uses: jdx/mise-action@v3
+        uses: jdx/mise-action@5228313ee0372e111a38da051671ca30fc5a96db # v3
         with:
           install: true
           cache: true
           working_directory: elixir
 
       - name: Cache deps and build
-        uses: actions/cache@v4
+        uses: actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830 # v4
         with:
           path: |
             elixir/deps

.github/workflows/pr-description-lint.yml

@@ -13,10 +13,10 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
 
       - name: Set up mise tools
-        uses: jdx/mise-action@v3
+        uses: jdx/mise-action@5228313ee0372e111a38da051671ca30fc5a96db # v3
         with:
           install: true
           cache: true