docs(nostr): consolidate migration epics #819 and #138
Create canonical consolidation plan and Telegram-Nostr bridge spec
under docs/nostr-migration/.
- CONSOLIDATION.md: establishes #819 as canonical parent epic,
maps scope boundaries, documents current implementation state
(Python stack + browser stack + infrastructure), lists action items
- TELEGRAM-NOSTR-BRIDGE-SPEC.md: highest-priority child issue spec
with requirements, architecture, implementation phases, and
acceptance criteria
Refs #862, #819, #138
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