chore(dependabot): cap open pull requests per update block

Adds `open-pull-requests-limit` to Dependabot update blocks that had no
cap, following the estate per-ecosystem cap doctrine (task #37).
No other line in the file is touched.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011eQ7hibx92N7fBDtwgReWk

Author: hyperpolymath

.github/dependabot.yml

@@ -8,3 +8,4 @@ updates:
       actions:
         patterns:
           - "*"
+    open-pull-requests-limit: 2