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.test.ts

@@ -38,7 +38,6 @@ describe("buildSpawnRequest", () => {
         worktree: {
           base_repo: "/home/user/project",
           branch: "feat/foo",
-          cleanup_on_kill: true,
           create_branch: true,
         },
       },
@@ -63,7 +62,6 @@ describe("buildSpawnRequest", () => {
         worktree: {
           base_repo: "/home/user/project",
           branch: "spawn-v2",
-          cleanup_on_kill: true,
           create_branch: false,
         },
       },