CHUNK #9: AI session storage layer (AgentController, AIStore, builders)

- Add packages/ai: AgentController (streaming + budget), SuggestionBuilder,
  NoteBuilder, AIStore (Dexie/IndexedDB), SuggestionMutation
- Fix createUlid to use monotonicFactory for reliable listSessions sort tiebreaker
- Extend outline system prompt with setting + characters context
- Add dexie dependency to packages/ai
- 188 tests passing (core 47, ui 5, editor 30, ai 71, exporters 22, web 13)

packages/core/src/ids.ts

@@ -1,10 +1,15 @@
-import { ulid } from "ulid";
+import { monotonicFactory } from "ulid";
 
 const ULID_REGEX = /^[0-7][0-9A-HJKMNP-TV-Z]{25}$/;
 
+// Monotonic factory: guarantees that successive calls produce ULIDs that sort
+// in creation order even within the same millisecond. This is important for
+// tiebreakers that rely on ULID ordering (e.g. reverse-chronological lists).
+const monotonicUlid = monotonicFactory();
+
 /** Generate a monotonic ULID. */
 export function createUlid(): string {
-  return ulid();
+  return monotonicUlid();
 }
 
 /** Validate a ULID string. */