feat: add Claude Desktop extension (.mcpb) for single-user stdio mode

Adds a `mcpb/` directory containing a Claude Desktop Extension bundle
that lets users install nextcloud-mcp-server from Claude Desktop without
needing HTTPS — the extension uses stdio transport (already supported via
`nextcloud_mcp_server/stdio.py`) so no network connection or TLS is
required.

Bundle contents:
- `manifest.json` — extension metadata, user_config fields for
  NEXTCLOUD_HOST, NEXTCLOUD_USERNAME, and NEXTCLOUD_PASSWORD (sensitive,
  stored in OS keychain)
- `run.sh` — shell wrapper that locates `uvx` across common install
  paths (official uv installer at ~/.local/bin, Homebrew at
  /opt/homebrew/bin, etc.) since Claude Desktop uses a restricted PATH
- `.gitignore` — excludes the built *.mcpb artifact

Build the distributable with:
    npm install -g @anthropic-ai/mcpb
    cd mcpb && mcpb pack

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

mcpb/.gitignore

@@ -0,0 +1 @@
+*.mcpb