feat(slack): add thinking_placeholder config option to disable "Thinking..." message

[SeHoJoo]

Summary

Adds `thinking_placeholder` config option to disable the `"Thinking..."` placeholder message in Slack channels.

UX motivation: With the current placeholder approach, users receive a push notification for `"Thinking..."` but not for the actual answer (message edits don't trigger Slack push). Disabling the placeholder causes the final response to arrive as a new message, which triggers a push notification on completion.

Changes:

• `config/config_channels.go` — adds `ThinkingPlaceholder *bool` to `SlackConfig`
• `channels/slack/channel.go` — reads config into `disableThinking` field
• `channels/slack/handlers.go` + `handlers_mention.go` — conditionally skips placeholder send (both DM and group @mention paths)
• `channels/slack/stream.go` — `StreamEnabled()` returns `false` when `disableThinking` is true; streaming requires a placeholder to edit into, so it is implicitly disabled together
• `channels/slack/factory.go` — adds `ThinkingPlaceholder` to `slackInstanceConfig` so DB-based channel instances (UI-configured) also respect the option
• `ui/web/.../channel-schemas.ts` — adds `thinking_placeholder` field; `dm_stream` and `group_stream` get `disabledWhen: thinking_placeholder=false` so they are grayed out with a hint in the UI
• `ui/web/.../channel-advanced-dialog.tsx` — adds `thinking_placeholder` to `STREAMING_KEYS` so it appears in Advanced Settings → Streaming section

Config (UI): Slack channel → Advanced Settings → Streaming → Thinking Placeholder toggle

Interaction with streaming options:
When `thinking_placeholder` is off, `dm_stream` / `group_stream` are also effectively disabled — streaming works by progressively editing the placeholder, so without one there is nothing to edit. `StreamEnabled()` enforces this explicitly. The UI reflects this by graying out the streaming toggles with a hint.

Default behavior (`thinking_placeholder` omitted or `true`) is fully unchanged.

Test plan

• Default behavior unchanged: omitting `thinking_placeholder` still sends `"Thinking..."` → edits with final response
• Toggle off in UI (Advanced Settings → Streaming) → no placeholder, final response arrives as new message
• Slack push notification received on response completion (not on placeholder)
• DM Streaming / Group Streaming toggles are grayed out when Thinking Placeholder is off
• `StreamEnabled()` returns `false` when `disableThinking` is true (streaming path not entered)
• Both DM and group @mention paths respect the config
• `go build ./...` passes
• `go vet ./internal/channels/slack/...` passes

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