Found by using it: the agent planted 18 beans, the client's stream dropped, and
those 18 plantings ended up in the garden with no change set behind them — real
changes with no way to undo them, which is the one guarantee this whole design
rests on.

The earlier fix detached the FAILURE path and reasoned that a cancelled context
is why fn failed. That missed the commoner case. An agent turn whose client
disconnects can still COMPLETE — the model finishes, the tools have already
written — and then the success path committed with a dead context, the write
failed, WithChangeSet returned an error, and the work was orphaned.

commitScope now detaches from cancellation itself, so every caller gets it. That
is the right home for the rule: by the time a commit runs, the data it describes
has already been written, so cancelling it cannot undo anything — it can only
lose the record of what happened. There is no path on which that is the
behaviour anyone wants.

Also added an SSE keep-alive, since the dropped connection is worth not having
in the first place: a model thinking between tool calls sends nothing for a
while, and an idle proxy will cut a quiet stream. A comment frame every 20s
keeps it open, and SSE ignores comment frames so the client is unaffected.

That introduced a second goroutine writing the response, so the event stream now
serializes writes behind a mutex — step events come from the agent's run
goroutine while the ticker writes from its own, and two goroutines writing a
ResponseWriter concurrently corrupts frames long before it crashes anything.
Verified under -race.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

@@ -64,2 +71,3 @@
 
-	send := openEventStream(c)
+	stream := openEventStream(c)
+	send := stream.send

@@ -66,0 +76,4 @@
+	// idle proxy will cut a quiet connection. Deferred so a panic in the run
+	// can't leak the ticker goroutine; stopping it twice is harmless.
+	stopBeat := stream.keepAlive(keepAliveInterval)
+	defer stopBeat()

@@ -157,0 +156,4 @@
+// commit runs, the data changes it describes have already been written — so
+// cancelling it cannot undo anything. It can only lose the record of what
+// happened and leave real changes with no way to undo them, which is the one
+// thing the whole change-set design exists to prevent.

@@ -205,0 +207,4 @@
+	// detached path as everything else — a REST client that hangs up right after
+	// its PATCH landed must not leave that change without history, and this is
+	// the path virtually every mutation takes.
+	auto := &changeScope{gardenID: gardenID, actorID: actorID, source: domain.SourceUI, summary: summary}