feat:copilot plugin to complement tmforge

[Hacks4Snacks]

What changed

• Add a nested tmforge Copilot plugin containing Strider and the threat-modeling and threat-modeling-tmforge skills.
• Bundle schemas, deterministic reporting, and validators without copying CLI binaries.
• Synchronize plugin and product versions through Release Please.
• Add CI regression coverage, discovery keywords, and installation/submission documentation.

Why

Make evidence-backed STRIDE threat modeling reusable across repositories. Markdown-only analysis works without tmforge; .tm7 workflows use a separately installed CLI.