fix(schema): move domain field to metadata.json for Claude Code compatibility

BREAKING FIX: Claude Code's marketplace schema validator rejects
unrecognized keys. The 'domain' field added in v8.0.0 caused
'Failed to load marketplace' errors on all 20 plugin entries.

- Removed 'domain' from marketplace.json (all 3 profiles)
- Removed 'domain' from all 20 plugin.json files
- Added 'domain' to metadata.json for all 20 plugins
- Updated validate-marketplace.sh to read from metadata.json
- Updated docs (CANONICAL-PATHS.md, CLAUDE.md) to reference metadata.json
- 12 new metadata.json files created (plugins without mcp_servers)
- 8 existing metadata.json files updated with domain field

Version: 9.1.2

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

plugins/saas-test-pilot/.claude-plugin/metadata.json

@@ -0,0 +1,3 @@
+{
+  "domain": "saas"
+}