Promptfoo drops stale caches from MCP eval discovery

This is a quiet but meaningful correctness fix: live evaluation state matters more than shaving a few reads from an MCP tool used by persistent AI assistants.

• –Eliminates stale evaluation lists in long-running Cursor, Claude Desktop, and other MCP sessions
• –Fresh reads ensure newly created, renamed, or deleted evaluations appear immediately
• –Removing default-config discovery caching reduces the risk of outdated project configuration
• –The trade-off is additional storage reads and potentially slightly slower repeated listing requests
• –The change reinforces Promptfoo’s role as an operational evaluation tool, not just a one-shot CLI