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/types/session.ts

@@ -846,7 +846,7 @@ export interface SessionState {
    * to `workingDir`, and a path resolved outside it goes through the attachment
    * routes like any other out-of-workspace path.
    */
-  cwd?: string;
+  paneCwd?: string;
   /** Remote execution metadata, present when this session runs over SSH through local tmux */
   remote?: SessionRemote;
   /** Docker execution metadata, present when this session runs inside a container via local tmux + docker exec */