docs: add IMPROVEMENTS.md with 9.3.0 release milestones

Add comprehensive improvement recommendations and release milestones:
- Security & stability improvements (token lifecycle, error handling)
- Database performance (indexes, N+1 fixes, caching)
- Feature completeness (guild, daily missions, tournament, tower)
- Operational excellence (health checks, metrics, logging)
- Discord bot enhancements
- Multi-version support audit
- Schema management infrastructure
- High-value packet implementations

Also add project roadmap section to CLAUDE.md referencing IMPROVEMENTS.md.

CLAUDE.md

@@ -389,3 +389,13 @@ The codebase supports multiple game client versions through:
 - Binary file compatibility (quests/scenarios in `bin/`)
 
 Primary focus: G10-ZZ (ClientMode), with varying support for older versions.
+
+## Project Roadmap
+
+See [IMPROVEMENTS.md](IMPROVEMENTS.md) for:
+
+- Prioritized improvement recommendations
+- Release milestones for 9.3.0
+- Database performance optimizations
+- Feature completeness tracking
+- Technical debt items