feat(node-manager): Task runtime core + AddNodeToGroup (Task layer increment 1)

[Apollon77]

First increment of the imperative, persisted, resumable Task layer on top of the reconciler. A Task orchestrates a multi-step, multi-node change by writing tier-1 desired-state intent and waiting on convergence gates — it never touches nodes directly; only the Reconciler does. Durable state is minimal, so tasks are restart-safe and offline-tolerant.

What

• TaskManagerBehavior — opt-in on the ServerNode root, attached via dynamic behaviors.require(ReconcilerBehavior) (no static .with; TaskManager pulls the Reconciler in itself). Registry (type → Task ctor), run(type, params, {externalId?}) (synchronous handle, internal deterministic id → idempotent re-issue), get/tasks/cancel, nonvolatile persistence ({type, params, phaseIndex, state, externalId?, addLog}), startup + on-register + on-online resume.
• TaskContext — two node-affecting verbs (setIntent/removeIntent, creates recorded into an add-log) + awaitGate(nodes, until) / awaitCommitted(items). Gates verify-reconcile the targeted peers, evaluate a predicate over their DesiredStateBehavior items, and suspend via per-gate ObserverGroup subscriptions (cross-peer → ObserverGroup, not reactTo) until it holds — parking while a node is unreachable, resuming on the reachability wake. Coalesced, abort-safe, no voided promises.
• Resume — re-instantiate non-terminal tasks by type, re-enter phaseIndex, re-run (phases re-entrant: setIntent upserts, gates re-read).
• Cancel = best-effort revert — reverse the add-log to deletePending, gate on removal → cancelled; offline peer ⇒ parks; terminal revert error ⇒ cancelFailed (reserved). Aborts an in-flight gate cleanly (cancelled, not failed).
• AddNodeToGroup — concrete task: provisions groupKey + groupKeyMap + endpointGroupMembership intents (converge; priority bands order keyset→group→membership), gates until all committed.

Scope

Increment 1 of 3. Add-log only — the full prior-state changeset + automatic hard-failure rollback + multi-node pre-flight admission + RemoveNodeFromGroup are increment 2; ipk kind + RotateGroupKey + 2-node rotation harness are increment 3.

Tests

• Unit: manager (require/run/dedup/external-id/persist), gates (resolve/wait/park-resume with concurrency), lifecycle (resume across restart, cancel reverse-revert, in-flight cancel).
• Integration (commissioned OnOffLightSwitchDevice.with(GroupsServer)): happy → member; park/resume; cancel → removed; restart-resume (real close + recreate-by-id → resumes to completed). All assert real device groupTable state.

Gates

build --clean ✓ · format ✓ · lint ✓ · node-manager 92/92 · node 1231/1231 (no regression). Per-task + whole-branch review: zero Critical/Important.

Base = node-manager (sub-PR; CI runs on umbrella #3948 → main).

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