Add serverless "MCP Apps on Azure Functions" skill + rework ui-widget-developer flow

[ericsche]

Summary

Extends the microsoft-365-agents-toolkit plugin with a new skill for building a
serverless MCP Apps server on Azure Functions (C# / .NET), and reworks the existing
ui-widget-developer skill so the key decisions come first and hosting options are explicit.

No new plugin is introduced — this adds one skill to an existing plugin and keeps all
plugin manifests in sync.

What's new — mcp-apps-azure-functions skill

A dedicated, serverless hosting path grounded in the official quickstart
(Build an MCP Apps server using Azure Functions)
and the Azure-Samples/remote-mcp-functions-dotnet template.

• SKILL.md — when to use / not use, prerequisites, and the end-to-end workflow:
  azd init template scaffold → build the UI app → local run (Azurite + Functions Core Tools)
  → verify with GitHub Copilot → deploy (azd provision / azd deploy) → connect remote → clean up.
• references/csharp-code-patterns.md — McpToolTrigger, McpResourceTrigger, McpMetadata,
  the ui.resourceUri contract, and how to add a new tool + widget.
• references/deployment.md — azd + Flex Consumption flow, Functions Core Tools alternative,
  Azurite, built-in Microsoft Entra auth, cost notes, and troubleshooting.

Reworked — ui-widget-developer skill

• Decisions first: the file now opens with (1) architecture choice and (2) hosting model,
  before any implementation detail.
• New "Choose your hosting model" section: a server-full vs serverless comparison, with
  serverless handing off to the new mcp-apps-azure-functions skill.
• MCP Apps prioritized over OAI Apps in the path selection (recommended default).
• Cleaner markdown hierarchy: telemetry demoted to a setup note, the clashing Architecture
  heading renamed to How it works, stray prose removed, and broken section anchors fixed.

Housekeeping

• Registered the new skill in the plugin README.md, PLUGINS.md, and AGENTS.md.
• Bumped microsoft-365-agents-toolkit 1.3.1 → 1.4.0 and enriched its description
  (serverless MCP Apps hosting), consistently across all five manifests:
  root marketplace.json, .claude-plugin/marketplace.json, and the plugin's
  .claude-plugin / .codex-plugin / .github/plugin plugin.json files.

Validation

• All five manifest JSON files parse successfully; no stale 1.3.1 references remain.
• New skill SKILL.md description is within the 1024-char frontmatter limit.
• No new markdown lint errors introduced (pre-existing anchor warnings in
  ui-widget-developer are unchanged).

Checklist

• Skill added under an existing plugin (plugins/microsoft-365-agents-toolkit/skills/mcp-apps-azure-functions/)
• SKILL.md with YAML frontmatter (name, description ≤ 1024 chars) + references
• Plugin manifests updated and version-bumped consistently (all 5 spots)
• README.md, PLUGINS.md, and AGENTS.md updated
• N/A — no new .mcp.json (skill-only addition, no new MCP server)

Files changed (12)

New

• plugins/microsoft-365-agents-toolkit/skills/mcp-apps-azure-functions/SKILL.md
• plugins/microsoft-365-agents-toolkit/skills/mcp-apps-azure-functions/references/csharp-code-patterns.md
• plugins/microsoft-365-agents-toolkit/skills/mcp-apps-azure-functions/references/deployment.md

Modified

• plugins/microsoft-365-agents-toolkit/skills/ui-widget-developer/SKILL.md
• plugins/microsoft-365-agents-toolkit/README.md
• PLUGINS.md, AGENTS.md
• marketplace.json, .claude-plugin/marketplace.json
• plugins/microsoft-365-agents-toolkit/.claude-plugin/plugin.json
• plugins/microsoft-365-agents-toolkit/.codex-plugin/plugin.json
• plugins/microsoft-365-agents-toolkit/.github/plugin/plugin.json