feat: Add format_mention tool for Slack channels

[Peyton-Spencer]

Context

PR #70 added format_mention as a Discord-only MCP tool (gated behind chatJid.startsWith('dc:')). It resolves display names to proper <@USER_ID> mention format using a user registry.

Slack has the same need — agents communicating in Slack channels need to <@U12345> mention users rather than plain-text @name.

Requirements

• Add format_mention support for Slack channels (JID prefix TBD based on Slack integration)
• Look up Slack user IDs from display names (Slack Web API users.list or similar)
• Return proper Slack mention format: <@USER_ID>
• Gated to Slack agents only, alongside the existing Discord gate

Notes

• The Slack skill currently exists at .claude/skills/slack/ as a standalone CLI tool
• User registry pattern from PR feat: User mentions with IDs and format_mention tool (Issue #66) #70 could be extended to support Slack user mappings
• May need a Slack-side user registry sync similar to the Discord user_registry.json

[Peyton-Spencer]

Blocked: Depends on Issue #100

This feature requires the Slack channel integration from upstream PR nanocoai#244 to be merged first.

Dependency chain:

1. ✅ Issue [Upstream PR #244] Slack channel support with Socket Mode #100 - Slack Socket Mode channel support (upstream PR feat: Add Slack channel support (RFS) nanocoai/nanoclaw#244)
2. ⏳ Issue feat: Add format_mention tool for Slack channels #71 - format_mention tool for Slack (this issue)

Implementation Notes

Once #100 merges, the implementation is straightforward:

// src/mcp/tools/format-mention.ts
if (chatJid.startsWith('dc:')) {
  // Discord implementation (existing)
  return formatDiscordMention(userName);
} else if (chatJid.startsWith('slack:')) { // ← Add this
  return formatSlackMention(userName);
}

async function formatSlackMention(userName: string): Promise<string> {
  const userId = await slackUserRegistry.lookup(userName);
  return userId ? `<@${userId}>` : userName;
}

User Registry Sync

Slack users can be synced via users.list API:

// Similar to Discord user_registry.json pattern
interface SlackUserRegistry {
  [displayName: string]: string; // display name → user ID (U12345)
}

Should be initialized when the Slack channel connects, similar to how Discord populates its registry.

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Recommendation: Mark as blocked and revisit after #100 is merged upstream.