docs: move loose specs out of .claude into docs/specs

102 KB of planning documents (website-builder-v2 spec, mobile app plan, mobile
API implementation, data snapshots) were sitting directly in .claude/ rather
than in docs/. They are reference material, not agent configuration.

.claude/ is now empty of markdown, so nothing in this repo is auto-loaded into
agent context except CLAUDE.md.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>