WaitBuffer is a thread-safe io.Writer that supports blocking until
accumulated output matches a substring or custom predicate. It
replaces ad-hoc safeBuffer/syncWriter types and time.Sleep-based
poll loops in tests with signal-driven waits.
- WaitFor/WaitForNth/WaitForCond for blocking on output
- Replace custom buffer types in cli/sync_test.go and
provisionersdk/agent_test.go
- Convert time.Sleep poll loops to require.Eventually/require.Never
in cli/ssh_test.go, coderd/activitybump_test.go,
coderd/workspaceagentsrpc_test.go, workspaceproxy_test.go, and
scaletest tests