fix: strip email domain from user identifier for environment endpoint

If preferred_username is an email (user@domain.com), extract just the
username part to match Qdrant collection naming (volatile_user).

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "core-api"
-version = "1.10.8"
+version = "1.10.9"
 description = "Core Code API - Infrastructure management and tools API"
 readme = "README.md"
 requires-python = ">=3.12"