chore: remove materialized plugin files from tracking

These agents/, commands/, and skills/ directories inside plugin folders
are generated by eng/materialize-plugins.mjs during CI publish and
should not be committed to the staged branch.

- Remove 185 materialized files from git tracking
- Add .gitignore rules to prevent accidental re-commits
- Update publish.yml to force-add materialized files despite .gitignore

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.gitignore

@@ -10,6 +10,11 @@ reports/
 # Generated files
 /llms.txt
 
+# Materialized plugin files (generated by CI via eng/materialize-plugins.mjs)
+plugins/*/agents/
+plugins/*/commands/
+plugins/*/skills/
+
 # Website build artifacts
 website/dist/
 website/.astro/