chore: Bump version to 0.19.0

Release highlights:
- Fix bd-160: JSONL integrity validation prevents export deduplication data loss
- Add comprehensive integration tests for export/import workflows
- Clear export_hashes on imports to prevent staleness
- Created bd-179 epic for remaining test coverage improvements

This release fixes a critical P0 bug that could cause silent data loss
when JSONL and export_hashes diverged after git operations.

.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "beads",
   "description": "AI-supervised issue tracker for coding workflows. Manage tasks, discover work, and maintain context with simple CLI commands.",
-  "version": "0.18.0",
+  "version": "0.19.0",
   "author": {
     "name": "Steve Yegge",
     "url": "https://github.com/steveyegge"