[ai] Extract CadenceEngine from Orchestrator (M3.5 Sub-3)

Parent Epic: #11022 (M3.5 Orchestrator Decomposition)

Context

As part of the M3.5 Orchestrator decomposition, we have successfully extracted TaskStateService (Sub-1) and ProcessSupervisorService (Sub-2). The next step is Sub-3: CadenceEngine extraction.

Currently, interval polling logic and cadence parsing (shouldRunIntervalTask, parseInterval, and boilerplate cycle-run logic) are tightly coupled inside ai/daemons/Orchestrator.mjs. We need to extract this into a dedicated CadenceEngine service.

Per OQ8 resolution in Discussion #11025, CadenceEngine must be a pure trigger-builder, NOT an execute-runner. It exposes a primitive like getIntervalTrigger({taskName, now, lastRunAt, intervalMs, reasonPrefix}).

Execution Plan

1. Target: Extract interval/cadence logic from ai/daemons/Orchestrator.mjs into ai/daemons/services/CadenceEngine.mjs.
2. Boundary: CadenceEngine returns a trigger object (or null if not due). It does not execute the task. Per-task coordinators decide "what work is due" by composing this primitive; the supervisor executes; the orchestrator wires.
3. Tests: Implement unit tests for CadenceEngine.spec.mjs covering interval parsing and trigger boundary conditions. Remove corresponding redundant tests from Orchestrator.spec.mjs while maintaining behavior.

Avoided Traps

• ❌ Making CadenceEngine an execute-runner: Doing a runIfDue shape would make CadenceEngine a mini-Orchestrator and violates OQ8.
• ❌ Direct state mutation: The trigger object will be passed down to ProcessSupervisor, which consumes TaskStateService for state updates.

Acceptance Criteria

• ai/daemons/services/CadenceEngine.mjs implemented as a pure trigger-builder.
• Orchestrator no longer contains parseInterval and shouldRunIntervalTask.
• CadenceEngine.spec.mjs authored and passing.