Add `subscribe(cb)` for change notifications (Q8)

[mogul]

Restored from slopsmith/slopsmith-plugin-stems#10 — original issue, opened by @byrongamatos on 2026-05-09.
[restored-from: slopsmith/slopsmith-plugin-stems#10]

From specs/001-stems-toggle/tasks.md — generated retrospectively by spec-kit.

Add subscribe(cb) for change notifications (Q8).

---

See specs/001-stems-toggle/spec.md and specs/001-stems-toggle/analyze.md for context.
This issue was filed in bulk; close as wontfix if no longer relevant.