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>

CHANGELOG.md

@@ -10,6 +10,8 @@ Docs: https://docs.clawd.bot
 - Markdown: add per-channel table conversion (bullets for Signal/WhatsApp, code blocks elsewhere). (#1495) Thanks @odysseus0.
 
 ### Fixes
+- Discord: limit autoThread mention bypass to bot-owned threads; keep ack reactions mention-gated. (#1511) Thanks @pvoo.
+- Gateway: accept null optional fields in exec approval requests. (#1511) Thanks @pvoo.
 - TUI: forward unknown slash commands (for example, `/context`) to the Gateway.
 - TUI: include Gateway slash commands in autocomplete and `/help`.
 - CLI: skip usage lines in `clawdbot models status` when provider usage is unavailable.