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.

src/session/session.ts

@@ -61,6 +61,8 @@ export function createSession(
   // H1: pass an existing id to RE-ATTACH to a surviving tmux session (e.g. after
   // a server restart). Default = a fresh id for a brand-new session.
   id: string = randomUUID(),
+  // M6: spawn directory for a new session ("new tab here"); defaults to homeDir.
+  cwd?: string,
 ): Session {
   // The id is injected into the shell env so Claude Code hooks know which tab an
   // event belongs to ($WEBTERM_SESSION) and where to POST ($WEBTERM_HOOK_URL, H2).
@@ -76,7 +78,7 @@ export function createSession(
     name: 'xterm-256color',
     cols: dims.cols,
     rows: dims.rows,
-    cwd: cfg.homeDir,
+    cwd: cwd ?? cfg.homeDir,
     env: {
       ...process.env,
       WEBTERM_SESSION: id,