feat(v0.3): M6 — new tab opens in the active tab's directory

- title-util.cwdFromOsc7: parse cwd from OSC 7 (file:// URL); 4 unit tests
- terminal-session: register OSC 7 handler → cwd getter; opts.cwd → sent in the
  attach frame on a fresh session
- protocol/types: ClientMessage attach.cwd (absolute path); validated
- session/manager/server: thread cwd → createSession spawn cwd (defaults homeDir)
- tabs: '+' (newTab) opens in the active tab's reported cwd, falls back to home
- Verified: server attach.cwd spawns there; cd /private/tmp → + → new tab pwd is
  /private/tmp. 212 tests green.

public/title-util.ts

@@ -23,3 +23,19 @@ export function folderFromTitle(title: string): string | null {
   if (slash !== -1) s = s.slice(slash + 1)
   return s || null
 }
+
+/**
+ * Parse the full cwd path from an OSC 7 sequence payload (M6).
+ *   data = "file://host/Users/me/dir"  →  "/Users/me/dir"
+ * Returns null if it isn't a file:// URL.
+ */
+export function cwdFromOsc7(data: string): string | null {
+  const m = /^file:\/\/[^/]*(\/.*)$/.exec(data.trim())
+  if (m === null) return null
+  const path = m[1] as string
+  try {
+    return decodeURIComponent(path)
+  } catch {
+    return path
+  }
+}