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/tabs.ts

@@ -151,7 +151,7 @@ export class TabApp {
 
   /* ── tab lifecycle ───────────────────────────────────────────── */
 
-  private addEntry(sessionId: string | null, customTitle: string | null): TabEntry {
+  private addEntry(sessionId: string | null, customTitle: string | null, cwd?: string): TabEntry {
     const entry: TabEntry = {
       session: null as unknown as TerminalSession,
       customTitle,
@@ -161,6 +161,7 @@ export class TabApp {
     }
     entry.session = new TerminalSession({
       sessionId,
+      ...(cwd !== undefined ? { cwd } : {}),
       onSessionId: () => this.persist(),
       // onActivity only fires for hidden (inactive) panes (see TerminalSession).
       onActivity: () => {
@@ -189,7 +190,9 @@ export class TabApp {
   }
 
   newTab(): void {
-    this.addEntry(null, null)
+    // M6: open the new tab in the active tab's current directory, if known.
+    const cwd = this.tabs[this.activeIndex]?.session.cwd ?? undefined
+    this.addEntry(null, null, cwd)
     this.persist()
     this.rebuild()
     this.activate(this.tabs.length - 1)