[abhinav]

Merge execution spreads local preparation, remote readiness, and
completion polling across the handler and scheduler adapter.
That makes following one merge require reading preflight and queue wiring
alongside its effects.

Give local preparation and trunk synchronization one per-command
workspace, and keep readiness, ordinary execution, and completion
with the operations they describe.
The handler retains preflight and workflow composition;
the scheduler retains per-change dependencies and failure propagation.
Preserve the existing preparation conditions, progress, polling,
and single-change merge behavior.

[abhinav]

This change is part of the following stack:

• submit: Preserve scope in native stack updates #1486
  • merge: Separate execution responsibilities #1484 ◀
    • mergequeue: Await preparation results #1488
      • merge: Use native ranges when available #1401
        • docs: Explain native GitHub stacks #1406

_{Change managed by git-spice.}