fix(slack): switch adapter to Socket Mode

[mperraillon]

Type of Change

• Fix - bug fix or security fix to source code

Description

src/channels/slack.ts wires createSlackAdapter for HTTP webhook mode (botToken + signingSecret), which requires a publicly reachable URL. But the rest of the codebase already expects Socket Mode:

• setup/verify.ts checks SLACK_BOT_TOKEN + SLACK_APP_TOKEN
• setup/migrate-v2/shared.ts preserves SLACK_APP_TOKEN from v1
• The /add-slack skill on main walks users through obtaining an App-Level Token (xapp-...) — see companion PR fix(skill/add-slack): switch to Socket Mode setup #2700

Result: a configured install has the right env vars (xoxb-... + xapp-...) but the adapter ignores SLACK_APP_TOKEN and falls back to webhook mode, which fails because no signing secret is set.

Fix

Read SLACK_APP_TOKEN, fail-fast if either token is missing, and pass mode: 'socket' + appToken to createSlackAdapter. The underlying @chat-adapter/slack package then opens a WebSocket to Slack instead of expecting inbound HTTP.

Heads-up for the reviewer

This change needs @chat-adapter/slack >= 4.27.0 (where the mode and appToken fields were added). The lockfile on this branch pins 4.26.0, so the code doesn't compile as-is.

I tried bumping @chat-adapter/slack to ^4.27.0 alone — it pulls in chat@4.27.0 transitively, which doesn't match the project's pinned chat@4.26.0. Fixing that by bumping chat to ^4.27.0 then surfaced a separate type issue: createChatSdkBridge's return type no longer exposes resolveChannelName, breaking the existing pattern used here and in telegram.ts.

So the version bump is bigger than a single-adapter fix — it's a coordinated @chat-adapter/* + chat SDK upgrade across all the channels. I left it out of this PR to keep the diff focused on the Socket Mode switch. Happy to bundle the SDK upgrade in if you'd prefer one PR.

Testing

• Manually verified in a local install (with the SDK upgrade applied locally): host log shows Slack auth completed, Slack socket mode connected, and Channel adapter started channel="slack" on startup. No public URL, no webhook server. DMs deliver end-to-end.
• Companion PR fix(skill/add-slack): switch to Socket Mode setup #2700 documents this setup on the user-facing side.