[DISCORDSH] Add slash commands and scheduled triggers for embeds

[h0lybyte]

Parent

Part of #7849

Description

Add Discord slash commands and optional scheduled triggers so users can request or auto-post Notice Board and Task Board embeds.

Acceptance Criteria

• /noticeboard <repo> slash command posts current blockers/stagnation
• /taskboard <repo> slash command posts current task status
• Optional cron/scheduled posting to designated channels
• Permission checks for who can trigger embeds