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/Services/GitSyncService.swift

@@ -852,9 +852,7 @@ class GitSyncService {
     }
 
     func sanitizeFilename(_ name: String) -> String {
-        // Remove invalid filename characters
-        let invalid = CharacterSet(charactersIn: "/\\:*?\"<>|")
-        return name.components(separatedBy: invalid).joined(separator: "-")
+        name.sanitizedForFilename()
     }
 
     static func extractProvider(from url: String) -> String {