fix: add conventional-commit prefix to dependabot PR titles (#11)

Dependabot's default PR titles ("Bump X from Y to Z") aren't
Conventional Commits format, so every dependabot PR failed
`pr-title-lint` the moment it went live on `main` — confirmed on #7, #8,
#9, and #10, all of which failed the title check despite being safe,
mergeable dependency bumps (all four retitled manually and re-verified
green in the meantime).

Adds `commit-message.prefix: chore` to both ecosystems in
`.github/dependabot.yml`, so future titles come out as `chore(deps):
bump X from Y to Z` — passes the lint, and gives `release-please` the
right commit type (a dependency bump is a `chore`, not a `fix` or
`feat`).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

Author: dconlan

.github/dependabot.yml

@@ -4,8 +4,12 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    commit-message:
+      prefix: "chore"
 
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
       interval: "weekly"
+    commit-message:
+      prefix: "chore"