fix(slack): propagate mediaLocalRoots through Slack send path

Restore Slack local file upload parity with CVE-era local media allowlist enforcement by threading `mediaLocalRoots` through the Slack send call chain.

- pass `ctx.mediaLocalRoots` from Slack channel action adapter into `handleSlackAction`
- add and forward `mediaLocalRoots` in Slack action context/send path
- pass `mediaLocalRoots` into `sendMessageSlack` for upload allowlist enforcement
- add changelog entry with attribution for this behavior fix

Co-authored-by: 2233admin <1497479966@qq.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>

src/channels/plugins/slack.actions.ts

@@ -15,7 +15,10 @@ export function createSlackActions(providerId: string): ChannelMessageActionAdap
         normalizeChannelId: resolveSlackChannelId,
         includeReadThreadId: true,
         invoke: async (action, cfg, toolContext) =>
-          await handleSlackAction(action, cfg, toolContext as SlackActionContext | undefined),
+          await handleSlackAction(action, cfg, {
+            ...(toolContext as SlackActionContext | undefined),
+            mediaLocalRoots: ctx.mediaLocalRoots,
+          }),
       });
     },
   };