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/partners/agents/amplitude-experiment-implementation.md

@@ -1 +0,0 @@
-../../../agents/amplitude-experiment-implementation.agent.md

plugins/partners/agents/apify-integration-expert.md

@@ -1 +0,0 @@
-../../../agents/apify-integration-expert.agent.md

plugins/partners/agents/arm-migration.md

@@ -1 +0,0 @@
-../../../agents/arm-migration.agent.md

plugins/partners/agents/comet-opik.md

@@ -1 +0,0 @@
-../../../agents/comet-opik.agent.md

plugins/partners/agents/diffblue-cover.md

@@ -1 +0,0 @@
-../../../agents/diffblue-cover.agent.md

plugins/partners/agents/droid.md

@@ -1 +0,0 @@
-../../../agents/droid.agent.md

plugins/partners/agents/dynatrace-expert.md

@@ -1 +0,0 @@
-../../../agents/dynatrace-expert.agent.md

plugins/partners/agents/elasticsearch-observability.md

@@ -1 +0,0 @@
-../../../agents/elasticsearch-observability.agent.md

plugins/partners/agents/jfrog-sec.md

@@ -1 +0,0 @@
-../../../agents/jfrog-sec.agent.md

plugins/partners/agents/launchdarkly-flag-cleanup.md

@@ -1 +0,0 @@
-../../../agents/launchdarkly-flag-cleanup.agent.md

plugins/partners/agents/lingodotdev-i18n.md

@@ -1 +0,0 @@
-../../../agents/lingodotdev-i18n.agent.md

plugins/partners/agents/monday-bug-fixer.md

@@ -1 +0,0 @@
-../../../agents/monday-bug-fixer.agent.md

plugins/partners/agents/mongodb-performance-advisor.md

@@ -1 +0,0 @@
-../../../agents/mongodb-performance-advisor.agent.md

plugins/partners/agents/neo4j-docker-client-generator.md

@@ -1 +0,0 @@
-../../../agents/neo4j-docker-client-generator.agent.md

plugins/partners/agents/neon-migration-specialist.md

@@ -1 +0,0 @@
-../../../agents/neon-migration-specialist.agent.md

plugins/partners/agents/neon-optimization-analyzer.md

@@ -1 +0,0 @@
-../../../agents/neon-optimization-analyzer.agent.md

plugins/partners/agents/octopus-deploy-release-notes-mcp.md

@@ -1 +0,0 @@
-../../../agents/octopus-deploy-release-notes-mcp.agent.md

plugins/partners/agents/pagerduty-incident-responder.md

@@ -1 +0,0 @@
-../../../agents/pagerduty-incident-responder.agent.md

plugins/partners/agents/stackhawk-security-onboarding.md

@@ -1 +0,0 @@
-../../../agents/stackhawk-security-onboarding.agent.md

plugins/partners/agents/terraform.md

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