Add per-conversation notes.md

Gives each conversation an opt-in, persistent memory file the model reads
automatically every turn and writes to on its own initiative via a fenced
```update-notes``` block in its reply — no per-write approval, matching the
Confab-as-CLAUDE.md-for-itself concept Rune wanted. /notes on|off|show,
files live in ~/Library/Application Support/oAI/notes/, embedded ID header
for future Git Sync compatibility. Adds DB migration v12.

oAI/Models/Conversation.swift

@@ -31,6 +31,8 @@ struct Conversation: Identifiable, Codable {
     var updatedAt: Date
     var primaryModel: String? // Primary model used in this conversation
     var folderId: UUID? // Folder this conversation is filed under, if any
+    var notesEnabled: Bool // Whether the per-conversation notes.md feature is on
+    var notesFilename: String? // Filename under Application Support/oAI/notes/, if notes have ever been created
 
     nonisolated init(
         id: UUID = UUID(),
@@ -39,7 +41,9 @@ struct Conversation: Identifiable, Codable {
         createdAt: Date = Date(),
         updatedAt: Date = Date(),
         primaryModel: String? = nil,
-        folderId: UUID? = nil
+        folderId: UUID? = nil,
+        notesEnabled: Bool = false,
+        notesFilename: String? = nil
     ) {
         self.id = id
         self.name = name
@@ -48,6 +52,8 @@ struct Conversation: Identifiable, Codable {
         self.updatedAt = updatedAt
         self.primaryModel = primaryModel
         self.folderId = folderId
+        self.notesEnabled = notesEnabled
+        self.notesFilename = notesFilename
     }
     
     var messageCount: Int {