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/gateway/protocol/schema/exec-approvals.ts

@@ -92,13 +92,13 @@ export const ExecApprovalRequestParamsSchema = Type.Object(
   {
     id: Type.Optional(NonEmptyString),
     command: NonEmptyString,
-    cwd: Type.Optional(Type.String()),
-    host: Type.Optional(Type.String()),
-    security: Type.Optional(Type.String()),
-    ask: Type.Optional(Type.String()),
-    agentId: Type.Optional(Type.String()),
-    resolvedPath: Type.Optional(Type.String()),
-    sessionKey: Type.Optional(Type.String()),
+    cwd: Type.Optional(Type.Union([Type.String(), Type.Null()])),
+    host: Type.Optional(Type.Union([Type.String(), Type.Null()])),
+    security: Type.Optional(Type.Union([Type.String(), Type.Null()])),
+    ask: Type.Optional(Type.Union([Type.String(), Type.Null()])),
+    agentId: Type.Optional(Type.Union([Type.String(), Type.Null()])),
+    resolvedPath: Type.Optional(Type.Union([Type.String(), Type.Null()])),
+    sessionKey: Type.Optional(Type.Union([Type.String(), Type.Null()])),
     timeoutMs: Type.Optional(Type.Integer({ minimum: 1 })),
   },
   { additionalProperties: false },