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/Utilities/Extensions/String+Extensions.swift

@@ -107,4 +107,12 @@ extension String {
         let endIndex = index(startIndex, offsetBy: length - trailing.count)
         return String(self[..<endIndex]) + trailing
     }
+
+    // MARK: - Filename Sanitization
+
+    /// Replaces characters invalid in filenames (on macOS/most filesystems) with "-".
+    func sanitizedForFilename() -> String {
+        let invalid = CharacterSet(charactersIn: "/\\:*?\"<>|")
+        return components(separatedBy: invalid).joined(separator: "-")
+    }
 }