Address Gadfly review on the agent runtime

The first finding breaks this PR's central promise and is the reason the review
was worth running.

The run context carries a timeout. When it fired, WithChangeSet's recovery path
tried to record what had already committed using that SAME dead context — which
fails, losing the history for changes that really happened. So a timed-out turn
left its partial work un-undoable, while the user-facing message cheerfully said
"anything I'd already changed is in History". That message was a lie in exactly
the case it was written for.

Both recovery paths (WithChangeSet and RevertChangeSet) now commit with
context.WithoutCancel. The commonest reason those paths run at all is a
cancelled or timed-out context, so using it to write the record of what it did
was self-defeating. There's a test that cancels mid-turn and asserts the partial
work is recorded, marked partial, and revertible.

turnSummary sliced bytes, so a message whose 120th byte fell inside a multibyte
character stored invalid UTF-8 in the history summary. Not hypothetical for text
people type. Trimmed by runes now, with a test using emoji.

RecordAgentExchange's failure was logged and swallowed, directly under a comment
claiming the user would see that their turn wasn't saved. The turn itself
succeeded, so Done still goes out — but with a warning saying the exchange
wasn't saved and won't survive a reload, because a clean "done" followed by a
conversation that has forgotten it is the quieter lie. It also now records with
a detached context, since the commonest reason that write fails is the client
having gone away, and the exchange is worth keeping either way.

AgentRunID was declared, documented as an executus join, and never set by
anything. It's set now, from a per-run id that's also logged at run start, so a
row in the history list has a thread back to the run that produced it — and the
comment describes that rather than a dependency this repo doesn't have.

Turn.History was populated and never read, left over from the client-held
history design that persistence replaced. Removed.

The SSE plumbing moved out of the handler into openEventStream, so agentChat
reads like the other decode/call/encode handlers in the package.

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

internal/service/revisions.go

@@ -99,7 +99,8 @@ type ChangeSetOptions struct {
 	Source string
 	// Summary is the one-line description shown in the history list.
 	Summary string
-	// AgentRunID joins this change set back to the executus run that produced it.
+	// AgentRunID joins this change set back to the agent run that produced it,
+	// so a change in the history list can be correlated with the run's log lines.
 	AgentRunID *string
 }
 
@@ -128,7 +129,11 @@ func (s *Service) WithChangeSet(ctx context.Context, actorID, gardenID int64, op
 		// which is exactly the situation undo exists for. Record what happened,
 		// mark the summary, and still report the failure.
 		sc.summary = partialSummary(sc.summary)
-		if _, cerr := s.commitScope(ctx, sc, nil); cerr != nil {
+		// Detached from cancellation on purpose. The commonest reason fn failed is
+		// that ctx was cancelled or timed out — and using that same dead context
+		// to record what committed would fail too, losing the history for changes
+		// that really happened. That is precisely the case this path exists for.
+		if _, cerr := s.commitScope(context.WithoutCancel(ctx), sc, nil); cerr != nil {
 			slog.Error("service: partial turn could not be recorded", "error", cerr, "garden", gardenID)
 		}
 		return nil, err
@@ -306,8 +311,10 @@ func (s *Service) RevertChangeSet(ctx context.Context, actorID, changeSetID int6
 		changes, conflict, err := s.applyInverse(ctx, r, applied)
 		if err != nil {
 			// Record what actually landed before surfacing the failure, so the
-			// partial revert is visible and undoable rather than orphaned.
-			if _, cerr := s.commitScope(ctx, sc, &target.ID); cerr != nil {
+			// partial revert is visible and undoable rather than orphaned. Detached
+			// from cancellation for the same reason as WithChangeSet's path: a
+			// timed-out context can't be used to write the record of what it did.
+			if _, cerr := s.commitScope(context.WithoutCancel(ctx), sc, &target.ID); cerr != nil {
 				slog.Error("service: partial revert could not be recorded", "error", cerr, "changeSet", changeSetID)
 			}
 			return nil, nil, err