refactor: rename preferences/curator to imprints/collector

[DannyMor]

Summary

• Rename all terminology: Preference → Imprint, CurationAgent → ImprintCollector, PreferenceStore → ImprintStore
• Rename curator/ module → collector/, preferences.py routes → imprints.py
• Update all MCP tool names (add_preference → add_imprint, etc.)
• Update API endpoints from /api/preferences to /api/imprints
• Update UI, README, pyproject.toml, CLI, setup text
• Qdrant collection name engram_preferences intentionally preserved
• All 67 tests pass

Test plan

• All unit tests pass
• Verify UI loads with Imprints terminology
• Verify MCP tools work with new names