feat: add the research skill

A small, model-invoked skill that spins up a background agent to
investigate a question against high-trust primary sources, then leaves a
single cited Markdown file wherever the repo keeps such notes.

Wires it into the plugin set, both READMEs (Model-invoked), a docs page,
and the ask-matt router, per the repo's promoted-skill conventions.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.claude-plugin/plugin.json

@@ -11,6 +11,7 @@
     "./skills/engineering/to-issues",
     "./skills/engineering/to-prd",
     "./skills/engineering/prototype",
+    "./skills/engineering/research",
     "./skills/engineering/domain-modeling",
     "./skills/engineering/codebase-design",
     "./skills/engineering/code-review",