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/message-utils.ts

@@ -16,6 +16,7 @@ export type DiscordChannelInfo = {
   name?: string;
   topic?: string;
   parentId?: string;
+  ownerId?: string;
 };
 
 type DiscordSnapshotAuthor = {
@@ -69,11 +70,13 @@ export async function resolveDiscordChannelInfo(
     const name = "name" in channel ? (channel.name ?? undefined) : undefined;
     const topic = "topic" in channel ? (channel.topic ?? undefined) : undefined;
     const parentId = "parentId" in channel ? (channel.parentId ?? undefined) : undefined;
+    const ownerId = "ownerId" in channel ? (channel.ownerId ?? undefined) : undefined;
     const payload: DiscordChannelInfo = {
       type: channel.type,
       name,
       topic,
       parentId,
+      ownerId,
     };
     DISCORD_CHANNEL_INFO_CACHE.set(channelId, {
       value: payload,