ci(dependabot): group GitHub Actions updates into one pull request

Every action is now pinned to a full commit hash with a `# vX.Y.Z`
comment, so Dependabot proposes an update for each new release of
each action, not just each new major version. With no `groups` for
the `github-actions` ecosystem, each monthly run would open a
separate pull request for every action with a new release. About ten
actions are in use, so one run could open about ten pull requests at
once, each running the whole CI matrix, and each would have to be
reviewed and merged on its own.

Add a `github-actions` group matching every action, so each monthly
run proposes all available action updates together in one pull
request. This is how the `pre-commit` ecosystem is already set up
here, and how gitoxide groups its `github-actions` updates.

The group applies only to version updates, which is the default when
`applies-to` is not set. Dependabot security updates, where enabled,
are still proposed individually.

`check-jsonschema` validates the configuration against the Dependabot
schema, and zizmor 1.31.1 at the `pedantic` level reports no findings
in it.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: EliahKagan

.github/dependabot.yml

@@ -6,6 +6,9 @@ updates:
     interval: monthly
   cooldown:
     default-days: 7
+  groups:
+    github-actions:
+      patterns: ["*"]
 
 - package-ecosystem: pre-commit
   directory: "/"