[slack] Use readable profile names in drafts

[Aismit]

Summary

• Update the Slack outgoing-message skill so slack_send_message_draft draft bodies use resolved, human-readable Slack profile names instead of raw user IDs or <@U...> mention markup.
• Keep explicit Slack mention syntax for direct sends, scheduled messages, and canvases where it is needed to create real mentions.
• Align the shared Slack markdown reference and outgoing-message skill metadata with the new draft-specific behavior.

Why

Drafts are review surfaces, and raw Slack IDs are hard for humans to read. This change makes draft text readable while preserving notification-capable mention syntax for actual Slack writes.

Simple Explanation

Before, the drafting guidance treated drafts like sent messages, so a draft could show something like <@U123456>. Now drafts should say the person's readable Slack name, while sent messages can still use Slack's special mention code when a real tag is needed.

Validation

• git diff --check
• ruby -e 'require "yaml"; YAML.load_file("plugins/slack/skills/slack-outgoing-message/agents/openai.yaml"); puts "ok"'

No runtime tests were run because this is a docs/skill-instructions-only change.