App-server: /v1/workflows* for GPUI workflow list, run, and cancel

GPUI Hmbown/codewhale-apps#622 Workflows module fails closed on missing HTTP. Apps probed and named `/v1/workflows*` as unavailable for GPUI.

Do not invent a second workflow executor in the desktop. Reconcile `crates/tui/src/runtime_api.rs`, `crates/tui/src/commands/groups/core/workflow.rs`, and the TUI `/workflow` surface against today's tip. If workflows are already served under another path (command registry, operate, or slash-only), name it. Otherwise serve list/read/run/cancel (draft-review-confirm-run) with Engine-owned run ids and no model turn for host cancel/status.

Related: Hmbown/codewhale-apps#622.

Read crates/tui/src/runtime_api.rs and crates/tui/src/commands/groups/core/workflow.rs, then inspect the TUI /workflow surface against today’s tip. Determine whether workflows already use another path such as the command registry, operate, or slash-only; otherwise the app-server should support list, read, run, and cancel with Engine-owned run IDs and no model turn for host cancel/status.

Written by the indexing model from the issue text.