## Description
This PR removes the last path in core that emits `ExecCommandBegin` /
`ExecCommandEnd` directly.
Every command execution now starts and completes through canonical
`ItemStarted` / `ItemCompleted(TurnItem::CommandExecution)`. The
existing `HasLegacyEvent` compatibility layer still fans out Begin/End
afterward, so raw core event consumers and legacy rollout replay keep
seeing the same events.
`UnifiedExecInteraction` is dormant today. Live unified exec uses
`UnifiedExecStartup` for command lifecycle and `TerminalInteraction` for
`write_stdin` and polling, so this is code cleanup rather than a current
product behavior change. The main win is the code-level invariant where
all core flows emit `TurnItem` instead of legacy events.
## What changed
- Removed the `UnifiedExecInteraction` branches that emitted legacy
command events directly.
- Routed every command source through the existing canonical
`CommandExecution` lifecycle and compatibility fanout.
Owen Lin
602dbb42dc