add cursor-based pull events: clide events --since <cursor> (T-223)

The one-shot request/response complement to the never-returning
`tail --events` stream — what an agent poll loop wants. The IPC server now
keeps a single global, arrival-ordered event log keyed by a monotonic
cursor (alongside the per-subsystem replay ring), bounded by eventLogDepth
with drop-oldest back-pressure (D-85: producer never blocks).

`clide events [--since <cursor>] [--filter X]` returns events after the
cursor, a high-water `cursor` to poll from next, and `gap: true` (+
oldestCursor) when the requested cursor predates the retained window so a
caller detects loss instead of silently missing dropped events. Repeated
polls neither drop nor duplicate. No on-disk persistence.

`events` is handled in the IPC server like `tail` (not the dispatcher);
added to the argv umbrella set. bindingWhen/CommandContribution untouched.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/src/cli/argv_to_request.dart

@@ -24,8 +24,10 @@ import 'package:clide/src/ipc/envelope.dart';
 import 'package:clide/src/ipc/schema_v1.dart';
 
 /// Umbrella commands — single-token names with no subsystem.verb
-/// split. Match the IDs the dispatcher exposes directly.
-const Set<String> _umbrellaCommands = {'status', 'tail', 'version', 'ping', 'capabilities'};
+/// split. Match the IDs the dispatcher exposes directly. `tail` and
+/// `events` are handled by the IPC server itself (streaming / cursor-pull
+/// event reads, T-129 / T-223) rather than the dispatcher.
+const Set<String> _umbrellaCommands = {'status', 'tail', 'events', 'version', 'ping', 'capabilities'};
 
 /// Sealed result of translating argv. Caller (the IPC server, or the
 /// C client wrapper in T-126) handles either branch.