Add world seeding, planner, and new slash commands
When a player creates a character and starts `storied play`, the DM used
to improvise from nothing — always the same generic opening. Now we
detect empty worlds (character exists, no session) and run a seeding
pass with Opus before play begins. The seeder builds outward from the
character's backstory: locations, NPCs, threads, and an opening moment.
Also adds `storied plan` to enrich thin entities near the player between
sessions, and `storied seed` for manual world seeding.
New slash commands:
- `/dm` for out-of-character messages to the DM
- `/note` to add notes to the character sheet mid-session
The DM system prompt now encourages shorter responses — a few paragraphs
instead of walls of text.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>