fix: track plugin-host source and add 35 sandbox security tests

Fix .gitignore 'plugins/' rule that was accidentally ignoring source
files in v2/src/lib/plugins/. Narrow to /plugins/ and /v2/plugins/
(runtime plugin directories only). Track plugin-host.ts (was written
but never committed) and add comprehensive test suite covering all 13
shadowed globals, this-binding, permission gating, API freeze, and
lifecycle management.

.gitignore

@@ -4,7 +4,8 @@ __pycache__/
 /CLAUDE.md
 v2/target/
 debug/
-plugins/
+/plugins/
+/v2/plugins/
 projects/
 .playwright-mcp/
 .audit/