Add exact module filtering to Query Store MCP results

[kendra-little]

Summary

• add an optional exact module-name filter to the Query Store MCP read
• apply the filter before cost ranking and result limits
• return module attribution with each result
• preserve existing unfiltered behavior across the Lite and Darling surfaces

Validation

• focused Query Store read tests
• Darling and Lite MCP contract tests
• Darling and Lite builds