Adds 'janitor' as a required system role alongside chat/orchestrator/distill.
Appears in the Model Registry UI so a cheap/fast model (Haiku, local Gemma)
can be assigned for upcoming session checkpoint compaction feature.
ROLE_DEFAULT_TOOLS: [] — no tools, pure text processing
_ROLE_LAST_RESORT: claude_cli fallback until a dedicated model is assigned
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>