refactor(maestro): drop cleanup_on_kill from the wire and the CLI

Nothing has honoured WorktreeSpec.cleanup_on_kill since #56, and after
#79 and #81 survival is decided by what is in a worktree, not by what
the spawn asked for. A field that lies is worse than a missing one.

Removes the field from the protocol, the --cleanup-on-kill flag from
the CLI, and the tests asserting its default.

web/src/spawnForm.ts

@@ -75,9 +75,6 @@ export function buildSpawnRequest(
     req.worktree = {
       base_repo: baseRepo,
       branch,
-      // The daemon always cleans up, guarded against unsaved work (#56);
-      // the wire field stays for older daemons.
-      cleanup_on_kill: true,
       // Existing-mode picks must check out as-is; the daemon creates
       // anything else with `worktree add -b` (#56/#61).
       create_branch: mode !== "existing",