Use glob for composite action directories in dependabot.yml

Replace the hardcoded list of four composite action directories with
`/.github/actions/*`. GitHub's `directories:` field supports single-level
glob matching, so adding a new composite action requires zero
dependabot.yml changes going forward.

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

Author: 2 people

.github/dependabot.yml

@@ -8,10 +8,7 @@ updates:
               patterns: ["*"]
     - package-ecosystem: github-actions
       directories:
-          - /.github/actions/bun_install
-          - /.github/actions/rust_toolchain
-          - /.github/actions/docker_compose_up
-          - /.github/actions/playwright_install
+          - /.github/actions/*
       schedule: { interval: weekly }
       groups:
           composite-action-internals: