chore: Bump version to 0.21.8

Updated all component versions:
- bd CLI: 0.21.7 → 0.21.8
- Plugin: 0.21.7 → 0.21.8
- MCP server: 0.21.7 → 0.21.8
- Documentation: 0.21.7 → 0.21.8

Generated by scripts/bump-version.sh

.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": "0.21.7",
+  "version": "0.21.8",
   "author": {
     "name": "Steve Yegge",
     "url": "https://github.com/steveyegge"