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.

oAITests/StringExtensionsTests.swift

@@ -0,0 +1,29 @@
+//
+//  StringExtensionsTests.swift
+//  oAITests
+//
+// SPDX-License-Identifier: PolyForm-Noncommercial-1.0.0
+// Copyright (C) 2026 Rune Olsen
+
+import Testing
+@testable import Confab
+
+@Suite("String.sanitizedForFilename")
+struct StringExtensionsTests {
+
+    @Test("Invalid filename characters are replaced with a dash")
+    func sanitizedForFilenameStripsInvalidChars() {
+        #expect("a/b\\c:d*e?f\"g<h>i|j".sanitizedForFilename() == "a-b-c-d-e-f-g-h-i-j")
+    }
+
+    @Test("A valid filename passes through unchanged")
+    func sanitizedForFilenameValidInput() {
+        #expect("My Chat 2026-01-15".sanitizedForFilename() == "My Chat 2026-01-15")
+    }
+
+    @Test("GitSyncService.sanitizeFilename delegates to the shared extension")
+    func gitSyncServiceDelegatesToExtension() {
+        let service = GitSyncService.shared
+        #expect(service.sanitizeFilename("a/b\\c:d*e?f\"g<h>i|j") == "a-b-c-d-e-f-g-h-i-j".sanitizedForFilename())
+    }
+}