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>

README.md

@@ -164,22 +164,21 @@ webclaw https://example.com/pricing --diff-with pricing-old.json
 
 webclaw ships with an MCP server for AI agents.
 
-```bash
-npx create-webclaw
-```
-
-Manual config:
+Zero-install — point any MCP client at the npx launcher:
 
 ```json
 {
   "mcpServers": {
     "webclaw": {
-      "command": "~/.webclaw/webclaw-mcp"
+      "command": "npx",
+      "args": ["-y", "@webclaw/mcp"]
     }
   }
 }
 ```
 
+Or run `npx create-webclaw` to auto-detect your AI tools and write their configs for you.
+
 Then ask your agent things like:
 
 ```text