Chore: Dependency pinning & Depdendabot introduction (#436)

1. Pins the Github Action workflow hashes under `.github/workflows` to
full-length commit SHAs.
2. Adds a base level `.github/dependabot.yml` config to keep these (and
other go/Docker) deps up to date.

## Motivation and Context
- Github's [security
guide](https://docs.github.com/en/actions/reference/security/secure-use#using-third-party-actions)
recommends using the immutable SHA of an action when consuming third
party actions.
- Depdendabot makes ongoing dep house keeping manageable.

.github/workflows/claude.yml

@@ -26,7 +26,7 @@ jobs:
       actions: read
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 #v4
         with:
           fetch-depth: 1