A wake-rule change whose gate.json write throws is never read back: it can report failure and then apply, or leave gate.json part-written #470

What's wrong

In src/gate/event-gate.ts (main e670363), writeConfig calls writeFileSync(this.configPath, …) and updates the in-memory config only if that call returns. That is right when the failure happens at open. But writeFileSync can also throw in two other places, and neither is read back:

• At close, after the whole text has landed. addGatePolicy/removeGatePolicy return an error (and so do wake_add_rule/wake_remove_rule), and the old rules stay in memory. The next reloadIfChanged then sees the changed text and applies it. The tool said "failed", yet the change is in force a second later.
• Partway through the write. gate.json is left part-written. If the host restarts before anyone fixes it, it starts from the host's starting rules, or from none: default "always", which wakes on every event.

Why it matters

Wake rules decide when an agent runs. Two outcomes are hard to diagnose after the fact: a change the agent was told had failed but that applies, and a file that becomes permissive on the next restart. #452 already reads gate.json back after a failed write in the startup reconcile. The mutation path doesn't.

How it was found

Flagged by the second reviewer during the review of #452. Confirmed by reading main's writeConfig and reloadIfChanged. This is not a regression from #452.

Suggested fix

On a write error, read gate.json back:

• If it holds exactly the intended text, commit the change as on success, or return an error that says the change landed.
• Otherwise, report that gate.json may be part-written.

Add mutation-path tests for "throws partway" and "throws at close".

Found during our review of #452: #452 (review)

_{Posted by the maintainer's desk assistant.}