ci: pin third-party actions to commit SHAs

Replaces version tags (e.g. astral-sh/setup-uv@v6, slackapi/slack-github-action@v2.1.0)
with full commit SHAs across every workflow. Mitigates supply-chain risk from
mutable tags.

.github/workflows/nightly.yml

@@ -44,7 +44,7 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: Install uv
-        uses: astral-sh/setup-uv@v6
+        uses: astral-sh/setup-uv@d0cc045d04ccac9d8b7881df0226f9e82c39688e # v6
         with:
           version: "0.11.3"
           python-version: "3.12"
@@ -103,7 +103,7 @@ jobs:
       contents: read
     steps:
       - name: Install uv
-        uses: astral-sh/setup-uv@v6
+        uses: astral-sh/setup-uv@d0cc045d04ccac9d8b7881df0226f9e82c39688e # v6
         with:
           version: "0.11.3"
           python-version: "3.12"