fix: keep an adopted session off the direct-PTY fallback (release review)

A wrapper that could not start fell back to a direct PTY: `claude
--dangerously-skip-permissions` or a server-account shell in /tmp on this
host, under a tab still carrying the foreign session's name, so someone who
opened an ssh or container session ended up typing into this machine.

startInteractive and startShell now fail the start for an adopted session
and leave `_useMux` on, so selecting the tab tries the wrapper again. The
`error` event carries the reason, since the browser's re-attach does not
read the route's answer. A session with no mux at all is refused the same
way.

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

docs/api-reference.md

@@ -1188,9 +1188,11 @@ session: respawn, Ralph (its config and the `fix-plan/write`, `fix-plan/read` an
 `ralph-prompt/write` file routes), auto-clear, auto-compact, auto-resume, custom model
 endpoints, prompt uploads and the image watcher. `stop` waits are a 400, and so are
 `blocked` waits unless Codeman reads that CLI's dialogs off its screen (codex), the git
-routes answer `unsupported` with `reason: "adopted"`, and `last-response` answers empty,
-since Codeman knows no transcript for it. Its `paneExit` stays absent, because the pane
-holds a `tmux attach` and not the agent. Reboot restore does not rebuild one.
+routes answer `unsupported` with `reason: "adopted"`, and
+`last-response` answers empty, since Codeman knows no transcript for it. Its `paneExit`
+stays absent, because the pane holds a `tmux attach` and not the agent. Reboot restore
+does not rebuild one. A wrapper that cannot start answers `OPERATION_FAILED` and leaves
+the tab to be tried again: Codeman never starts a local process in its place.
 
 ## Approvals Inbox