Add Sent messaging plugin

[sent-dm]

What

Adds the official sentdm/sent-plugin repository as an external plugin in the GitHub Copilot plugin marketplace.

Why

Sent gives developers a single plugin for sending and monitoring SMS, WhatsApp, and RCS messages, managing contacts and templates, querying account and delivery analytics, and building compliant messaging workflows. The external source contains a hosted MCP configuration and 13 specialist skills.

Impact

Adds one MIT-licensed external marketplace entry at version 0.1.0; no runtime code in this repository changes.

Verification

• marketplace JSON parses successfully
• plugin names remain unique
• the Claude marketplace symlink resolves to the updated GitHub marketplace manifest
• the public source manifest matches name sent, version 0.1.0, and license MIT
• Awesome Copilot Vally lint passes all 13 source skills and its Agent Plugins v1.0.0 manifest check passes
• git diff --check passes