fix: ignore vanished cgroup create events

[joejulian]

Root cause

The raw watcher can receive a directory-create event after a short-lived cgroup has already been removed. watchDirectory then returns ENOENT, which currently propagates even though that create/remove ordering is expected.

Change

Only the dynamic ContainerAdd path treats os.IsNotExist returned by watchDirectory as this race and returns without emitting a container event. Every other watch or read error remains unchanged.

Tests

• Covers a cgroup directory disappearing before its create event is processed.
• Verifies that a non-ENOENT error (ENOTDIR) is still returned.

Related historical context: #3464. This narrowly addresses the disappeared-directory create-event race and does not claim to resolve every symptom discussed there.

Validation

• Focused race-enabled tests
• 43-package lib suite
• Canonical make test
• Repository-root go vet
• Exact golangci-lint v2.6.2
• Boilerplate, gofmt, and diff checks