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>
