feat(chat): add system prompt editor & personas (M9.2)

Named personas with system prompt, model preference, and accent colour.
Persona pill selector above chat input, editor modal for create/edit/delete.
Default persona auto-applied to new conversations. Persona system prompt
prepended to AI context.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/core/src/types/message.ts

@@ -54,4 +54,6 @@ export interface Conversation {
   branchPoint?: string
   /** IDs of child branches forked from this conversation */
   childBranchIds?: string[]
+  /** ID of the persona applied to this conversation */
+  personaId?: string
 }