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.test.ts

@@ -377,12 +377,63 @@ describe("discord mention gating", () => {
       resolveDiscordShouldRequireMention({
         isGuildMessage: true,
         isThread: true,
+        botId: "bot123",
+        threadOwnerId: "bot123",
         channelConfig,
         guildInfo,
       }),
     ).toBe(false);
   });
 
+  it("requires mention inside user-created threads with autoThread enabled", () => {
+    const guildInfo: DiscordGuildEntryResolved = {
+      requireMention: true,
+      channels: {
+        general: { allow: true, autoThread: true },
+      },
+    };
+    const channelConfig = resolveDiscordChannelConfig({
+      guildInfo,
+      channelId: "1",
+      channelName: "General",
+      channelSlug: "general",
+    });
+    expect(
+      resolveDiscordShouldRequireMention({
+        isGuildMessage: true,
+        isThread: true,
+        botId: "bot123",
+        threadOwnerId: "user456",
+        channelConfig,
+        guildInfo,
+      }),
+    ).toBe(true);
+  });
+
+  it("requires mention when thread owner is unknown", () => {
+    const guildInfo: DiscordGuildEntryResolved = {
+      requireMention: true,
+      channels: {
+        general: { allow: true, autoThread: true },
+      },
+    };
+    const channelConfig = resolveDiscordChannelConfig({
+      guildInfo,
+      channelId: "1",
+      channelName: "General",
+      channelSlug: "general",
+    });
+    expect(
+      resolveDiscordShouldRequireMention({
+        isGuildMessage: true,
+        isThread: true,
+        botId: "bot123",
+        channelConfig,
+        guildInfo,
+      }),
+    ).toBe(true);
+  });
+
   it("inherits parent channel mention rules for threads", () => {
     const guildInfo: DiscordGuildEntryResolved = {
       requireMention: true,