feat(events): thread idempotencyKey through the event envelope

[coopbri]

Summary

Completes the exactly-once story started in the vortex fix (vortex-worker #56 / vortex-api #81). The worker now dedups on idempotencyKey ?? id, but events emitted through @omnidotdev/providers never carried an idempotencyKey, so every producer using this package fell back to dedup-by-id — a fresh UUID per emit, so at-least-once retries double-processed downstream.

• Added optional idempotencyKey?: string to EventInput (documented: unique per logical event, distinct from correlationId which is shared across related events).
• Included it in the Iggy envelope (iggy.ts) and the HTTP POST body (http.ts), the two paths that reach Vortex. vortex-api's /events endpoint already accepts the field.

Additive and backward-compatible: the field is optional, so existing producers are unchanged and pick up dedup only when they choose to set it.

Testing

• tsc / biome clean; 33 events tests pass (2 new: the POST body forwards idempotencyKey when set, omits it when unset).

Note

Consumers repin @omnidotdev/providers to adopt the field; no immediate fleet repin is required since the behavior is opt-in.

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