selfupdate: the staging folder keeps what it needs and no more (R-1368 part 2) — not released

Each good update left its staged binary in <state dir>/selfupdate for ever
(49 files / 740 MB on demo-hp). After the wrapper's commit returned success,
MaybeCommit now removes every staged binary but the running version's.

Nothing is removed while a pending marker exists or cannot be read, at a
plain start, or after a rollback. Only a regular file with the exact name
Execute writes is removed. The rollback reads pending.json and
felhom-agent.prev, no staged binary: pinned by a test on the wrapper's text
and by a test that runs the wrapper's real commit and rollback verbs in a
sandbox. Red seen three times.

No version bump, no build, no vouch. /usr/local/bin/felhom-agent.bak-* is
not reached (root-owned; R-1368 part 4).

Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

internal/selfupdate/executor.go

@@ -103,6 +103,9 @@ func (e *Executor) Execute(ctx context.Context, op string, params json.RawMessag
 	}
 	e.logger.Debug("agent_update: op invariants passed (semver + sha format)", "version", p.Version)
 
+	// The staged file is this call's from the download to the hand-off: a prune (prune.go) waits for it.
+	stagingMu.Lock()
+	defer stagingMu.Unlock()
 	dir := stagingDir(e.stateDir)
 	if err := os.MkdirAll(dir, 0o750); err != nil {
 		return fmt.Errorf("agent_update: staging dir: %w", err)