Complete GAP-61: Anonymous Gifts (Emma Goldman) + is_private field

GAP-61 Implementation:
- Made agent_id Optional in Listing model and validation
- Added anonymous field (bool) for attribution-free gifts
- Created migration 006_add_anonymous_gifts.sql
- Added find_anonymous_gifts() repository method
- Added GET /vf/listings/community-shelf endpoint
- Added indexes for efficient community shelf queries

Additional Changes:
- Added is_private field to listings (opt-out sharing)
- Created migration 003_add_is_private_to_listings.sql
- Updated inter-community-sharing proposal

Philosophical alignment: Emma Goldman - "The most violent element
in society is ignorance." Anonymous gifts enable pure generosity
without expectation of reciprocity or recognition.

🤖 Generated with Claude Code
Co-Authored-By: Claude <noreply@anthropic.com>

Author: lizTheDeveloper