Remove the raw-GET completion-subscriber preservation once the SDK round-trips the field

[jeremy]

Follow-up to #538 / #540, gated on basecamp/basecamp-sdk#355 (and the broader merge-safe update work planned there).

#540 shipped an interim mitigation: because the SDK's Todo model drops completion_subscribers in todoFromGenerated, todos update preserves subscribers via a raw GET of the flat /todos/{id}.json route (completionSubscriberIDs in internal/commands/todos.go), failing closed before any PUT.

Once the CLI bumps to an SDK release where the Todo model carries CompletionSubscribers (and ideally a merge-safe Update/Edit):

1. Delete completionSubscriberIDs and both preservation call sites (typed-merge and raw-clear branches).
2. Source subscribers from existingTodo like every other preserved field — or collapse the whole update onto the SDK's Edit if available.
3. Keep everything else: --notify-on-completion / --no-notify-on-completion, resolveCompletionSubscriberIDs, set/clear semantics (clear-by-omission), tests, SKILL.md docs, and the smoke scenario. The preservation tests must stay green across the swap — they are the regression net for the plumbing change.

[jeremy]

Unblocked: basecamp/basecamp-sdk#375 merged (merge commit d15f023f64dc) — merge-safe Update, Edit closure, and Replace across all six SDKs, with UpdateTodo → ReplaceTodo in the generated layer. Next step here: make bump-sdk to a pin at or past d15f023f, collapse todos update onto the SDK's merge-safe path, and delete completionSubscriberIDs plus both preservation call sites. The #540 preservation tests are the regression net for the swap.