feat(contract-validator): add breaking change detection hook (#230)

Implements PostToolUse hook to warn about breaking interface changes:
- Detects changes to plugin.json, hooks.json, .mcp.json, agents/*.md
- Compares with git HEAD to find removed/changed elements
- Warns on: removed hooks, changed matchers, removed MCP servers
- Warns on: plugin name changes, major version bumps
- Non-blocking, configurable via CONTRACT_VALIDATOR_BREAKING_WARN

Depends on #229 (SessionStart auto-validate infrastructure).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/contract-validator/hooks/hooks.json

@@ -0,0 +1,21 @@
+{
+  "hooks": {
+    "SessionStart": [
+      {
+        "type": "command",
+        "command": "${CLAUDE_PLUGIN_ROOT}/hooks/auto-validate.sh"
+      }
+    ],
+    "PostToolUse": [
+      {
+        "matcher": "Edit|Write",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "${CLAUDE_PLUGIN_ROOT}/hooks/breaking-change-check.sh"
+          }
+        ]
+      }
+    ]
+  }
+}