feat: add Slack adapter with multi-platform ChatAdapter architecture

[dogzzdogzz]

Summary

• Introduces ChatAdapter trait + AdapterRouter, decoupling session management, streaming, and reactions from any specific chat platform
• Refactors existing Discord support to implement the new trait (no behavior change)
• Adds Slack adapter using Socket Mode (WebSocket, no public URL needed) with auto-reconnect
• Extracts shared image/audio processing into media.rs for cross-adapter reuse
• Both adapters can run simultaneously, sharing one SessionPool with namespaced keys
• Updates Helm chart to support Slack deployment (backward compatible)

Closes #93. Implements Phase 1–3 of RFC #94.

Changes

File	Change
src/adapter.rs	New — ChatAdapter trait, ChannelRef/MessageRef/SenderContext, AdapterRouter
src/slack.rs	New — SlackAdapter + Socket Mode event loop
src/media.rs	New — shared image resize/compress + STT download (extracted from discord.rs)
src/discord.rs	Refactored — DiscordAdapter implements ChatAdapter, delegates to router
src/reactions.rs	Decoupled from serenity — uses Arc<dyn ChatAdapter>
src/format.rs	Added shared shorten_thread_name()
src/config.rs	discord now optional, added SlackConfig
src/main.rs	Multi-adapter startup, Slack as background task
config.toml.example	Added [slack] section with docs
charts/openab/values.yaml	Added slack config per agent (enabled, botToken, appToken, allowedChannels, allowedUsers)
charts/openab/templates/configmap.yaml	Conditionally generates [discord] and/or [slack] sections
charts/openab/templates/secret.yaml	Stores slack-bot-token + slack-app-token alongside discord token
charts/openab/templates/deployment.yaml	Injects SLACK_BOT_TOKEN + SLACK_APP_TOKEN env vars
charts/openab/templates/NOTES.txt	Added Slack setup instructions

Design Decisions

• Socket Mode over Events API — no public URL needed, mirrors Discord's persistent WebSocket model
• Session keys namespaced by platform (discord:xxx, slack:xxx) to prevent collision
• Sender context wrapping centralized in AdapterRouter (not duplicated per adapter)
• Image/audio processing shared via media.rs with optional auth_token param (Discord CDN is public, Slack files need Bearer token)
• Unicode → Slack emoji mapping in adapter (Slack reactions API requires short names like eyes not 👀)
• Slack user name resolution via users.info API (Discord provides names in message payload, Slack only gives user ID)
• Thread follow-ups — Slack message events with thread_ts handled without requiring re-mention (parity with Discord)
• Helm backward compatible — existing Discord-only values work unchanged; [discord] section now conditional (omitted when no botToken)

Helm Usage

# Discord only (unchanged)
helm install openab openab/openab \
  --set agents.kiro.discord.botToken="$DISCORD_TOKEN" \
  --set-string 'agents.kiro.discord.allowedChannels[0]=CHANNEL_ID'

# Slack only
helm install openab openab/openab \
  --set agents.kiro.slack.enabled=true \
  --set agents.kiro.slack.botToken="$SLACK_BOT_TOKEN" \
  --set agents.kiro.slack.appToken="$SLACK_APP_TOKEN" \
  --set-string 'agents.kiro.slack.allowedChannels[0]=C0123456789'

# Both simultaneously
helm install openab openab/openab \
  --set agents.kiro.discord.botToken="$DISCORD_TOKEN" \
  --set-string 'agents.kiro.discord.allowedChannels[0]=DISCORD_CHANNEL' \
  --set agents.kiro.slack.enabled=true \
  --set agents.kiro.slack.botToken="$SLACK_BOT_TOKEN" \
  --set agents.kiro.slack.appToken="$SLACK_APP_TOKEN"

Test plan

• cargo build — zero warnings
• cargo test — 26 tests pass
• cargo clippy — clean
• helm template — Discord-only, Slack-only, and both modes validated
• Discord-only config works identically (no regression)
• Slack-only config starts Socket Mode and responds to @mentions
• Discord + Slack simultaneous config works
• Thread follow-up in Slack (reply without @mention)
• Reactions display correctly in Slack
• Image attachments processed in Slack
• Auto-reconnect on Socket Mode disconnect

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