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/server.ts

@@ -229,7 +229,7 @@ export function startServer(cfg: Config): { close(): Promise<void> } {
 
         let session
         try {
-          session = manager.handleAttach(ws, msg.sessionId, dims, Date.now())
+          session = manager.handleAttach(ws, msg.sessionId, dims, Date.now(), msg.cwd)
         } catch (err: unknown) {
           // M4: spawn failure — send exit(-1) and close only this connection.
           const reason =