feat(chat): validate and support omp /handoff from Chat #800

Problem

A user reports that omp's handoff workflow is unavailable or impractical in Chat, and currently switches to Terminal for native commands.

Baseline reviewed: v0.4.6, also compared with current main. The exact omp version, invocation and failing step were not provided, so the end-to-end failure is not yet reproduced.

What is established

• The omp builtin completion list in server/commands.ts omits handoff.
• The Composer sends manually entered text, including unlisted slash commands, as written. Missing completion does not establish that /handoff cannot be submitted.
• There is no dedicated handoff responder or end-to-end handoff coverage. Generic prompt handling may cover parts of the flow; that needs testing.

Proposed work / acceptance

1. Pin the omp version and reproduce the exact invocation, including any argument/no-argument variants it supports.
2. Identify which summary, editor, selection or confirmation steps require native terminal interaction.
3. Add completion and Chat interaction only for supported steps. If a step must use Terminal, offer a clear same-pane handoff to Terminal without losing the user's draft.
4. Verify cancel/error handling and that Chat binds to the correct conversation after a successful native handoff; never choose a transcript merely by modification time.
5. Exercise local and remote panes in an owned test session. Prompt answers must use the existing server-owned navigation semantics.

Adding a name to the completion list alone is not sufficient evidence that the workflow works.

Where: Chat and prompt cards. Workaround: use Terminal for omp's native workflow.