feat: add NanoClaw server-side skill and update docs

Adds the complete server-side MCP implementation as a NanoClaw
feature skill (nanoclaw-skill/). Includes source code, modification
intents, and step-by-step SKILL.md for installation. README updated
to cover both server (NanoClaw) and client (pi.dev) setup.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nanoclaw-skill/modify/src/channels/index.ts

@@ -0,0 +1,2 @@
+// mcp
+import '../mcp-server.js';

nanoclaw-skill/modify/src/channels/index.ts.intent.md

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+# Intent: Add MCP server import
+
+Add `import '../mcp-server.js';` to the channel barrel file so the MCP
+server self-registers as a channel with the channel registry on startup.
+
+This is an append-only change — existing import lines for other channels
+must be preserved.