feat(v0.2): tab title = current folder (from OSC title)

- title-util.ts: folderFromTitle() parses 'user@host:~/path' → last folder
  segment (home stays ~); pure, dependency-free, 7 unit tests
- terminal-session.ts: onTitleChange → folderFromTitle so the tab auto-names
  to the cwd folder and updates live on cd
- Browser-verified: ~ → cd project → 'web-terminal' → cd docs → 'docs' → cd ~ → '~'.
  198 tests green, typecheck + build ok.

public/title-util.ts

@@ -0,0 +1,25 @@
+/**
+ * public/title-util.ts — derive a short tab title from a terminal OSC title.
+ *
+ * Pure, dependency-free (no xterm/DOM) so it's unit-testable in node.
+ */
+
+/**
+ * Derive the **current folder** from the terminal's OSC title.
+ *
+ * Most shells emit titles like `user@host:~/path/to/dir` (zsh/bash default).
+ * Take the part after the last ':' (the path), then its last segment (the
+ * folder name). `~` (home) stays `~`. Returns null for an empty title; falls
+ * back to the raw-ish basename when there's no `user@host:` shape (e.g. a
+ * running command title).
+ */
+export function folderFromTitle(title: string): string | null {
+  let s = title.trim()
+  if (!s) return null
+  const colon = s.lastIndexOf(':')
+  if (colon !== -1) s = s.slice(colon + 1).trim()
+  s = s.replace(/\/+$/, '') // strip trailing slash
+  const slash = s.lastIndexOf('/')
+  if (slash !== -1) s = s.slice(slash + 1)
+  return s || null
+}