fix: name the pane directory field paneCwd and apply the review items for #740

`cwd` beside the fixed `workingDir` reads ambiguously on a public API surface, and
`paneCwd` says where the value comes from, like `paneExit`. Also pin the field on
the payload the browser really receives (getSessionStateWithRespawn), and give the
pane-directory text its own architecture-invariants paragraph.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/web/server.ts

@@ -2866,10 +2866,10 @@ export class WebServer extends EventEmitter {
       // or empty read and carries a blank `#{pane_current_path}` forward, so an
       // `undefined` here means an authoritative read listed no single directory
       // for the session (split into several panes, or gone from the socket).
-      const cwdChanged = getPaneCurrentPath ? session.setCwd(getPaneCurrentPath(muxName)) : false;
+      const paneCwdChanged = getPaneCurrentPath ? session.setPaneCwd(getPaneCurrentPath(muxName)) : false;
       if (session.setPaneExit(getPaneExit(muxName))) {
         this.persistSessionState(session);
-      } else if (!cwdChanged) {
+      } else if (!paneCwdChanged) {
         continue;
       }
       this.broadcastSessionStateDebounced(session.id);