refactor: migrate plugins to Claude Code spec format

- Move plugin manifests from .github/plugin/ to .claude-plugin/
- Convert items[] to Claude Code spec fields (agents, commands, skills)
- Rename tags to keywords, drop display/featured/instructions from plugins
- Delete all symlinks and materialized files from plugin directories
- Add eng/materialize-plugins.mjs to copy source files into plugin dirs at publish time
- Add .github/workflows/publish.yml for staged->main publishing
- Update CI triggers to target staged branch
- Update validation, creation, marketplace, and README generation scripts
- Update CONTRIBUTING.md and AGENTS.md documentation
- Include all new content from main (polyglot-test-agent, gem-browser-tester,
  fabric-lakehouse, fluentui-blazor, quasi-coder, transloadit-media-processing,
  make-repo-contribution hardening, website logo/gradient changes)

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

plugins/azure-cloud-development/agents/azure-logic-apps-expert.md

@@ -1 +0,0 @@
-../../../agents/azure-logic-apps-expert.agent.md

plugins/azure-cloud-development/agents/azure-principal-architect.md

@@ -1 +0,0 @@
-../../../agents/azure-principal-architect.agent.md

plugins/azure-cloud-development/agents/azure-saas-architect.md

@@ -1 +0,0 @@
-../../../agents/azure-saas-architect.agent.md

plugins/azure-cloud-development/agents/azure-verified-modules-bicep.md

@@ -1 +0,0 @@
-../../../agents/azure-verified-modules-bicep.agent.md

plugins/azure-cloud-development/agents/azure-verified-modules-terraform.md

@@ -1 +0,0 @@
-../../../agents/azure-verified-modules-terraform.agent.md

plugins/azure-cloud-development/agents/terraform-azure-implement.md

@@ -1 +0,0 @@
-../../../agents/terraform-azure-implement.agent.md

plugins/azure-cloud-development/agents/terraform-azure-planning.md

@@ -1 +0,0 @@
-../../../agents/terraform-azure-planning.agent.md