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

@@ -9,6 +9,7 @@ updates:
       actions:
         patterns:
           - "*"
+    open-pull-requests-limit: 2
 
   # Rust dependencies. Keep every Cargo project in one update entry so the
   # open-PR limit applies to the fleet as a whole, rather than once per bot.

robot-repo-automaton/.github/dependabot.yml

@@ -8,6 +8,7 @@ updates:
       actions:
         patterns:
           - "*"
+    open-pull-requests-limit: 2
 
   - package-ecosystem: "cargo"
     directory: "/"
@@ -16,13 +17,16 @@ updates:
     ignore:
       - dependency-name: "*"
         update-types: ["version-update:semver-patch"]
+    open-pull-requests-limit: 0
 
   - package-ecosystem: "npm"
     directory: "/"
     schedule:
       interval: "daily"
+    open-pull-requests-limit: 3
 
   - package-ecosystem: "pip"
     directory: "/"
     schedule:
       interval: "daily"
+    open-pull-requests-limit: 3