fix(plugin-slack): drop unnecessary as-unknown cast on user WebClient — Greptile P2

`WebClient` in this file is aliased to `App["client"]` (line 26),
which is the same `@slack/web-api` `WebClient` class imported here
as `SlackWebClient`. The two types are structurally identical, so
the double `as unknown as WebClient` was masking — not bridging —
type compatibility. Drop the `unknown` indirection so any future
divergence between the Bolt-wrapped client and the direct import
surfaces as a type error at the cast site instead of being silently
swallowed.

Inline comment expanded to record the invariant for future readers.

Verification:
- bunx tsc --noEmit -p plugins/plugin-slack/tsconfig.json: clean
- bunx vitest run plugins/plugin-slack/src/: 17/17 pass

Follow-up to Greptile P2 on PR #7876 (post-merge).

Author: 2-A-M

plugins/plugin-slack/src/service.ts

@@ -732,8 +732,12 @@ export class SlackService extends Service implements ISlackService {
       // session is needed. Only constructed when a user token is
       // configured. Routing decisions in getOutboundClient() consult
       // account.role to decide which client receives each call.
+      // `WebClient` is the alias for `App["client"]`, which is the
+      // same `@slack/web-api` `WebClient` class imported here as
+      // `SlackWebClient` — a direct `as WebClient` cast is correct
+      // and a future type divergence will surface as an error here.
       const userClient = account.userToken
-        ? (new SlackWebClient(account.userToken) as unknown as WebClient)
+        ? (new SlackWebClient(account.userToken) as WebClient)
         : null;
 
       const state: SlackAccountRuntime = {