Support model-owned token budget defaults

[copyberry]

Support model-owned token budget defaults

What changed

• Add token-budget settings to model catalog messages and apply them when the
  feature is enabled without explicit token-budget configuration.
• Keep explicit user settings authoritative, reject invalid catalog defaults,
  and preserve resolved defaults in exported config locks.
• Manage context-window guidance through world state so it updates once when
  the active model changes while retaining prior conversation history.

Testing

• Cover catalog defaults, explicit overrides, disabled features, invalid
  values, config-lock replay, and model switching.