feat: add Slack channel support

[Peyton-Spencer]

Summary

• Add SlackChannel implementation using @slack/bolt Socket Mode (WebSocket, no public URL needed)
• Workspace-level shared context across Slack channels (mirrors Discord's server-level pattern)
• Full feature parity with Discord: image attachments, bot mention translation, DM auto-trigger, share_request tool, JID-based routing (slack:{channelId} / slack:dm:{channelId})
• Setup skill at .claude/skills/add-slack/SKILL.md with prerequisites, OAuth scopes, event subscriptions, and troubleshooting

Files changed

• New: src/channels/slack.ts, .claude/skills/add-slack/SKILL.md
• Modified: package.json, src/config.ts, src/types.ts, src/db.ts, src/index.ts, src/ipc.ts, src/container-runner.ts, src/task-scheduler.ts, container/agent-runner/src/index.ts, container/agent-runner/src/ipc-mcp-stdio.ts

Test plan

• bun run build compiles cleanly
• Add SLACK_BOT_TOKEN + SLACK_APP_TOKEN to .env, restart — verify "Slack bot connected via Socket Mode" in logs
• Send @BotName hello in a Slack channel — verify message stored in DB
• Register a Slack channel via main agent — verify group folder + CLAUDE.md created
• Verify workspace backfill — check servers/{workspace-slug}/CLAUDE.md exists
• Trigger share_request from Slack agent — verify it appears in WhatsApp
• React on WhatsApp to approve — verify main agent wakes up and fulfills
• Cross-group send_message to Slack channel — verify delivery
• Rebuild container: container builder stop && container builder rm && container builder start && ./container/build.sh

🤖 Generated with Claude Code

[github-actions]

This PR adds a skill while also modifying source code. A skill PR should not change source files—the skill should contain instructions for Claude to follow. See /convert-to-docker for an example.

If you're fixing a bug or simplifying code, please submit that as a separate PR.

See CONTRIBUTING.md for details.

[Peyton-Spencer]

CI Failure Analysis

The CI check is failing because this PR violates the repository's contribution guidelines by mixing two types of changes:

1. Source code changes: Adding Slack channel support (src/channels/slack.ts, config changes, etc.)
2. Skill file: Adding setup instructions (.claude/skills/add-slack/SKILL.md)

According to CONTRIBUTING.md, skill PRs should only contain instructions for Claude to follow, not modify source files.

Solution

This PR should be split into two separate PRs:

PR 1: Slack Channel Implementation

• All source code changes (src/channels/slack.ts, config.ts, types.ts, etc.)
• Package.json dependencies
• Container runner changes
• Remove the .claude/skills/add-slack/ directory

PR 2: Slack Setup Skill

• Only the .claude/skills/add-slack/SKILL.md file
• Should be submitted after PR 1 is merged
• Contains instructions for Claude to help users set up Slack

I'll work on creating these split PRs now.

[Peyton-Spencer]

Closing in favor of split PRs per CONTRIBUTING.md:

• feat: add Slack channel support #5: Slack channel implementation (source code only)
• docs: add Slack setup skill #6: Slack setup skill (skill file only)

This ensures compliance with the guideline that skill PRs should not modify source files.