fix(slack): resolve reaction user display names

[akkadaska]

Summary

Fixes Slack reaction events so event.user.userName and event.user.fullName are resolved from Slack user profile data instead of always being the raw Slack user ID.

This reuses the Slack adapter's existing cached lookupUser() path and falls back to the user ID when lookup fails.

Fixes #521

Test plan

• Manually verified in Slack that chat.onReaction() receives the reacting user's display name in event.user.userName instead of the raw Slack user ID.
• Manually verified in Slack that chat.onReaction() receives the reacting user's real name in event.user.fullName instead of the raw Slack user ID.
• Manually verified that event.user.userId still contains the raw Slack user ID.
• pnpm --filter @chat-adapter/slack exec vitest run src/index.test.ts -t "resolves reaction user display name|resolves parent thread_ts"
• pnpm --filter @chat-adapter/slack typecheck
• pnpm --filter @chat-adapter/slack build
• pnpm validate

Checklist

• All commits are signed and verified
• pnpm validate passes
• Changeset added (or N/A — see CONTRIBUTING.md)
• Documentation updated (or N/A)

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