feat(channels): add Discord adapter (#625)

[molecule-ai]

Summary

Implements the Discord channel adapter so Community Manager (and any other workspace) can send messages to Discord and receive inbound slash commands.

• platform/internal/channels/discord.go — DiscordAdapter implementing ChannelAdapter:
  • ValidateConfig: requires webhook_url starting with https://discord.com/api/webhooks/ (matches the Slack SSRF-guard pattern)
  • SendMessage: POSTs {"content": text} to the webhook; splits messages >2000 chars at newline/space boundaries (Discord hard limit)
  • ParseWebhook: handles type-1 PING (returns nil — router layer must respond {"type":1}), type-2 APPLICATION_COMMAND, type-3 MESSAGE_COMPONENT; extracts text as /commandname option1 option2; prefers member.user over user (guild vs DM)
  • StartPolling: returns nil (Discord uses Interactions webhook, not long-polling)
• platform/internal/channels/discord_test.go — 20 unit tests (ValidateConfig × 6, SendMessage error paths × 2, ParseWebhook × 5, StartPolling, registry × 2, splitMessage × 4)
• platform/internal/channels/registry.go — register "discord": &DiscordAdapter{}
• .env.example — document DISCORD_WEBHOOK_URL

Org-template change (separate repo)

community-manager/workspace.yaml in molecule-ai-org-template-molecule-dev will be updated in a companion PR to wire DISCORD_WEBHOOK_URL into the Community Manager's channel config. Community Manager is the natural owner — its role description explicitly calls out Discord triage.

Setup instructions

1. Discord Server → channel → Edit Channel → Integrations → Webhooks → New Webhook → Copy Webhook URL
2. Add as global secret: POST /settings/secrets {"key":"DISCORD_WEBHOOK_URL","value":"https://discord.com/api/webhooks/..."}
3. For inbound slash commands: create a Discord Application, set Interactions Endpoint URL to https://<platform-host>/webhooks/discord

Test plan

• go test ./platform/internal/channels/... passes (all 20 new Discord tests + existing)
• GET /channels/adapters returns {"type":"discord","display_name":"Discord"} in list
• Create a workspace channel via Canvas or API with type=discord and a valid webhook URL; verify POST /workspaces/:id/channels/:id/test sends a test message to Discord

Closes #625

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