feat(pi-sentinel): migrate delivery to pi-dispatch

Replace the private pending-map/coalescing flush in the extension with
publishes into the shared @parke.dev/pi-dispatch queue. Sentinel events
are published with a sentinel: source prefix and mapped priorities
(gate ALL PASS -> escalation; complete/elapsed/merged/closed ->
completion; everything else -> info), and cancellation suppresses
queued items by source prefix. ensureDelivery(pi) wires the single
delivery path; coalescedMessage/activeSnapshot are deleted as dead
code. Bumps to 0.3.0.