feat(pi-dispatch): add unified session notification queue

New @parke.dev/pi-dispatch library: a per-process singleton queue
(registered on globalThis) that extensions publish notifications into,
with exactly one delivery path into the pi session. Items accumulate
while the agent is busy and drain as a single batched message on idle
after a 2s coalescing window, grouped escalation -> completion -> info.
Repeat ids fold to the latest state with an (xN) count, cancelled
sources are suppressed by prefix, and triggerTurn is set only when a
wake-urgent item is in the batch.

Implements docs/plans/2026-08-14-persistent-subagents.md section 4.