chore: group dependabot updates and use conventional commit prefix

Bundle all minor/patch bumps per ecosystem into a single PR to keep
noise manageable (~3 PRs/month instead of 10+). Major bumps still
get individual PRs since they may break APIs.

Commit messages now use the `chore(deps)` conventional-commit prefix
to match the repo's existing style.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/dependabot.yml

@@ -4,13 +4,34 @@ updates:
     directory: "/"
     schedule:
       interval: "monthly"
+    commit-message:
+      prefix: "chore(deps)"
+      include: "scope"
+    groups:
+      minor-and-patch:
+        patterns: ["*"]
+        update-types: ["minor", "patch"]
 
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
       interval: "monthly"
+    commit-message:
+      prefix: "chore(deps)"
+      include: "scope"
+    groups:
+      minor-and-patch:
+        patterns: ["*"]
+        update-types: ["minor", "patch"]
 
   - package-ecosystem: "docker"
     directory: "/"
     schedule:
       interval: "monthly"
+    commit-message:
+      prefix: "chore(deps)"
+      include: "scope"
+    groups:
+      minor-and-patch:
+        patterns: ["*"]
+        update-types: ["minor", "patch"]