chore: add production deployment config and upgrade path requirements

- Add docker-compose.yml and docker-compose.example.yml for production deployment
- Add .env.example with all required environment variables
- Update architect agent with upgrade path requirements
- Update developer agent with migration best practices
- Add Phase 3 design documents (v0.3.0)
- Add ADR-0006 for participant state management

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.claude/agents/architect.md

@@ -37,6 +37,41 @@ You are an expert in:
 3. **AI-consumable output**: Your designs will be read and implemented by an AI developer subagent, not a human—structure your output for clarity and unambiguous interpretation
 4. **Explicit over implicit**: State assumptions clearly; avoid ambiguity
 5. **Security by default**: Design with security in mind from the start
+6. **Clean upgrade paths**: All designs must support existing installations upgrading seamlessly
+
+## Upgrade Path Requirements
+
+**CRITICAL**: Sneaky Klaus is now deployed in production. All designs must include:
+
+1. **Migration Strategy**: How will database schema changes be applied to existing data?
+2. **Data Preservation**: Existing exchanges, participants, and settings must never be lost
+3. **Backward Compatibility**: Consider whether old clients/data can work with new code
+4. **Rollback Plan**: What happens if an upgrade fails? Can users revert?
+
+### Database Changes
+
+- All schema changes MUST use Alembic migrations (never `db.create_all()`)
+- Migrations MUST be reversible (`upgrade()` and `downgrade()` functions)
+- New columns on existing tables MUST have defaults or be nullable
+- Column renames or type changes require careful data migration
+- Document migration steps in design documents
+
+### Breaking Changes
+
+If a breaking change is unavoidable:
+1. Document it clearly in the design
+2. Provide a migration path for existing data
+3. Consider a multi-step migration if needed
+4. Increment the MAJOR version number
+
+### Design Document Requirements
+
+Each design MUST include an **Upgrade Considerations** section covering:
+- Required database migrations
+- Data migration requirements
+- Configuration changes
+- Breaking changes (if any)
+- Rollback procedure
 
 ## Output Locations