docs: add federation roadmap with batched execution plan

Comprehensive audit of the federation implementation identified:
- Solid data plumbing (multi-WS, origin routing, asset normalization)
- Missing on-ramps (can't join remote servers, no discovery, no UI indicators)
- 4 bugs (stale voice state, logout cleanup, reply-to normalization, HTTPS assumption)

Roadmap defines 6 batches for incremental execution:
- Batch 1: Bug fixes
- Batch 2: Password enforcement (same password across all instances)
- Batch 3: Cross-instance server joining (invite URL parsing, remote join routing)
- Batch 4: Phase 5 UI (instance grouping, badges, username collision display)
- Batch 5-6: Deferred (explore system, cross-instance friends)

.gitignore

@@ -49,3 +49,7 @@ packages/web/src/**/*.js
 # Misc
 .cache/
 tmp/
+
+# Local files
+Gemini Starter.rtf
+System Prompt.rtf