Add fork-PR-target hook (verbatim from bang-game #63) + battery CI gate

Propagates the PreToolUse enforce-pr-target.sh backstop (byte-identical to
claridtimo/bang-game master) that denies any `gh pr create` not explicitly
aimed at claridtimo/*, guarding against accidental PRs on the greyhavens
upstream fork parent. Ships the 103-case regression battery and a CI job
(.github/workflows/hook-battery.yml) that gates it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E3cmNbMM8gGpprBy8kcNuV

.claude/settings.json

@@ -0,0 +1,16 @@
+{
+  "hooks": {
+    "PreToolUse": [
+      {
+        "matcher": "Bash",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "$CLAUDE_PROJECT_DIR/.claude/hooks/enforce-pr-target.sh",
+            "timeout": 10
+          }
+        ]
+      }
+    ]
+  }
+}