feat(mcp): add @webclaw/mcp zero-install launcher as the canonical MCP command

`npx -y @webclaw/mcp` downloads the prebuilt webclaw-mcp binary (verified
against the release SHA256SUMS), caches it, and runs it as an MCP stdio server.
This makes webclaw introspectable in MCP registries (Glama/Smithery/MCP
registry) and installable in any MCP client with no Rust build.

- packages/webclaw-mcp: zero-dep launcher (.mjs) + package.json + README
- README/CLAUDE/examples/smithery: mcpServers config -> npx -y @webclaw/mcp
- registry: point the MCP-registry entry (server.json) at @webclaw/mcp, the
  real stdio server, instead of create-webclaw (a scaffolder); drop
  create-webclaw's mcpName claim so ownership isn't duplicated
- CHANGELOG: unreleased entry

create-webclaw stays as the auto-detect installer. .mcp.json is intentionally
left pointing at the local install path until @webclaw/mcp is published to npm.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

examples/README.md

@@ -273,14 +273,15 @@ webclaw https://example.com  # Automatically detects and uses proxies.txt
 ## MCP Server (AI Agent Integration)
 
 ```bash
-# Start the MCP server (stdio transport)
-webclaw-mcp
+# Run the MCP server (stdio transport) — no install needed
+npx -y @webclaw/mcp
 
 # Configure in Claude Desktop (~/.config/claude/claude_desktop_config.json):
 # {
 #   "mcpServers": {
 #     "webclaw": {
-#       "command": "/path/to/webclaw-mcp",
+#       "command": "npx",
+#       "args": ["-y", "@webclaw/mcp"],
 #       "env": {
 #         "WEBCLAW_API_KEY": "wc_your_key"  // optional, enables cloud fallback
 #       }
@@ -288,7 +289,7 @@ webclaw-mcp
 #   }
 # }
 
-# Available tools: scrape, crawl, map, batch, extract, summarize, diff, brand, research, search
+# Available tools: scrape, search, crawl, map, batch, extract, summarize, diff, brand, research, lead, lead_batch, list_extractors, vertical_scrape
 ```
 
 ## Real-World Recipes

examples/mcp-web-scraping/README.md

@@ -16,7 +16,8 @@ The installer detects supported MCP clients and can write the config for you.
 {
   "mcpServers": {
     "webclaw": {
-      "command": "~/.webclaw/webclaw-mcp",
+      "command": "npx",
+      "args": ["-y", "@webclaw/mcp"],
       "env": {
         "WEBCLAW_API_KEY": "wc_your_key"
       }