feat(runtime-host): bind ScheduledTasks to immutable Connection identity

[me2seeks]

Parent tracker

• [Tracking] Explicit multi-account provider connections across Runtime Host surfaces #3852
• Depends on the Session exact-target contract

Problem

ScheduledTask agent-run effects currently persist a model slug and model without immutable Connection identity. If account A is deleted and account B later reuses A's slug, a durable task can silently execute with B.

ScheduledTask is a separate durable aggregate and should not expand the Session identity PR.

Desired outcome

Persist and execute every new or replaced ScheduledTask agent-run target as exact connectionId + connectionSlug + model.

Acceptance criteria

• New and replaced agent-run effects freeze the exact Connection ID, slug, and model.
• Task idempotency fingerprints include Connection ID.
• Execution revalidates ID+slug+model before allocating a Session, AgentRun, or provider effect.
• Task-created Sessions receive the exact target.
• Delete A/reuse slug B never dispatches B credentials or creates execution state for an A-bound task.
• Legacy slug-only tasks remain readable and non-effect editable but fail closed before execution until explicitly recreated/reselected.
• Core codec, workflow persistence, Runtime Host coordinator/protocol, idempotency, corruption, and real two-client race tests cover the invariant.
• The closed ScheduledTask wire shape advances the compatibility epoch once from the then-current main value.

Non-goals

• Session legacy adoption
• CLI/Desktop account pickers
• Onboarding or OAuth
• Automatic account fallback

Generated-by: Codex