feat(slack): add native text streaming support

Adds support for Slack's Agents & AI Apps text streaming APIs
(chat.startStream, chat.appendStream, chat.stopStream) to deliver
LLM responses as a single updating message instead of separate
messages per block.

Changes:
- New src/slack/streaming.ts with stream lifecycle helpers using
  the SDK's ChatStreamer (client.chatStream())
- New 'streaming' config option on SlackAccountConfig
- Updated dispatch.ts to route block replies through the stream
  when enabled, with graceful fallback to normal delivery
- Docs in docs/channels/slack.md covering setup and requirements

The streaming integration works by intercepting the deliver callback
in the reply dispatcher. When streaming is enabled and a thread
context exists, the first text delivery starts a stream, subsequent
deliveries append to it, and the stream is finalized after dispatch
completes. Media payloads and error cases fall back to normal
message delivery.

Refs:
- https://docs.slack.dev/ai/developing-ai-apps#streaming
- https://docs.slack.dev/reference/methods/chat.startStream
- https://docs.slack.dev/reference/methods/chat.appendStream
- https://docs.slack.dev/reference/methods/chat.stopStream

src/config/zod-schema.providers-core.ts

@@ -480,6 +480,7 @@ export const SlackAccountSchema = z
     chunkMode: z.enum(["length", "newline"]).optional(),
     blockStreaming: z.boolean().optional(),
     blockStreamingCoalesce: BlockStreamingCoalesceSchema.optional(),
+    streaming: z.boolean().optional(),
     mediaMaxMb: z.number().positive().optional(),
     reactionNotifications: z.enum(["off", "own", "all", "allowlist"]).optional(),
     reactionAllowlist: z.array(z.union([z.string(), z.number()])).optional(),