fix(discord): autoThread ack reactions + exec approval null handling (#1511)

* fix(discord): gate autoThread by thread owner

* fix(discord): ack bot-owned autoThreads

* fix(discord): ack mentions in open channels

- Ack reactions in bot-owned autoThreads
- Ack reactions in open channels (no mention required)
- DRY: Pass pre-computed isAutoThreadOwnedByBot to avoid redundant checks
- Consolidate ack logic with explanatory comment

* fix: allow null values in exec.approval.request schema

The ExecApprovalRequestParamsSchema was rejecting null values for optional
fields like resolvedPath, but the calling code in bash-tools.exec.ts passes
null. This caused intermittent 'invalid exec.approval.request params'
validation errors.

Fix: Accept Type.Union([Type.String(), Type.Null()]) for all optional string
fields in the schema. Update test to reflect new behavior.

* fix: align discord ack reactions with mention gating (#1511) (thanks @pvoo)

---------

Co-authored-by: Wimmie <wimmie@tameson.com>
Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/discord/monitor/threading.ts

@@ -14,6 +14,7 @@ export type DiscordThreadChannel = {
   name?: string | null;
   parentId?: string | null;
   parent?: { id?: string; name?: string };
+  ownerId?: string | null;
 };
 
 export type DiscordThreadStarter = {
@@ -63,6 +64,7 @@ export function resolveDiscordThreadChannel(params: {
     name: channelInfo?.name ?? undefined,
     parentId: channelInfo?.parentId ?? undefined,
     parent: undefined,
+    ownerId: channelInfo?.ownerId ?? undefined,
   };
 }