feat(sync): replace WS+DO transport with unified HTTP sync

Replace the WebSocket + Cloudflare Durable Object architecture with a
single POST /sync endpoint. The CLI now operates autonomously with local
state (tasks.json) and syncs bidirectionally via adaptive-interval HTTP
polling (3s watching, 60s idle).

- Remove transport_ws, transport_hybrid, transport_http (~2,600 lines)
- Add SyncClient with adaptive interval loop
- Add LocalState for CLI-side task persistence
- Add TaskStateFromUpdate() helper (DRY)
- Extract finalize() to deduplicate processTask/processTaskRetry
- Consolidate shortID() into agent.ShortID (was in 3 packages)
- Wire GetActiveCount so `unarr status` shows active tasks
- Remove poll_interval, heartbeat_interval, ws_url from config
- Simplify ProgressReporter (sync replaces direct HTTP reporting)

go.mod

@@ -11,7 +11,6 @@ require (
 	github.com/fatih/color v1.19.0
 	github.com/getsentry/sentry-go v0.44.1
 	github.com/google/uuid v1.6.0
-	github.com/gorilla/websocket v1.5.3
 	github.com/huin/goupnp v1.3.0
 	github.com/olekukonko/tablewriter v1.1.4
 	github.com/spf13/cobra v1.10.2
@@ -69,6 +68,7 @@ require (
 	github.com/google/btree v1.1.3 // indirect
 	github.com/google/go-cmp v0.7.0 // indirect
 	github.com/google/pprof v0.0.0-20260302011040-a15ffb7f9dcc // indirect
+	github.com/gorilla/websocket v1.5.3 // indirect
 	github.com/huandu/xstrings v1.5.0 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/klauspost/cpuid/v2 v2.3.0 // indirect