refactor(lexicon): remove membership record references
Membership tracking is AppView-only data, not atProto records.
Changes:
- Removed membershipUri field from community.get viewerState
- Updated member field description to clarify it's AppView-computed
- Removed membership lexicon file (already deleted)
- Removed membership test data files (already deleted)
Rationale:
- Membership/reputation is indexed from user activity, not explicit records
- No need for AT-URI reference to non-existent record type
- Clarifies that membership status is computed by AppView, not stored in repo
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Co-Authored-By: Claude <noreply@anthropic.com>