Build a cross-agent post-edit validation plugin

The Meridian web team wants a deterministic check to run after an AI coding agent edits a file. The same behavior must work across supported agents:

• Trigger after Edit or Write tool calls.
• Run npm run validate:changed from the consuming project.
• Keep the hook entrypoint inside the plugin so it ships with the plugin.
• Do not modify Claude Code, Cursor, or any other agent's configuration directly.

Create a Tessl plugin named meridian/validate-after-edit under validate-after-edit/. Produce the complete plugin files on disk, ready for tessl plugin lint and tessl plugin pack. Use the smallest composition that implements the deterministic lifecycle behavior; do not add a skill or rule that merely describes the check.