feat(terminal-types): support custom default and ordering

[nkarpov]

Summary

• add optional default and order fields to terminal type manifests (type.json)
• allow custom terminal types to become the default session type (default: true)
• add deterministic ordering for session types:
  • explicit order (ascending)
  • then default
  • then name/id
• keep built-in terminal as fallback default when no custom default is specified
• update frontend session-type model to honor backend ordering/default metadata
• document new fields in README and terminal-types docs

Behavior

• default: true on a custom type makes it the default session type
• if multiple custom types set default: true, one is selected by ordering rules and a warning is logged
• order is an integer priority (lower = earlier)
• ordered types appear before unordered types

Validation

• npm run check
• npm test
• npm run build