feat(mcp): add ponytail-mcp, an MCP server for the ruleset (#91)

* feat(mcp): add ponytail-mcp server (prompt + tool)

* test(mcp): cover mode resolution and instruction text

* Report resolved MCP mode

ponytail-mcp/README.md

@@ -0,0 +1,46 @@
+# ponytail-mcp
+
+An MCP server that serves Ponytail's lazy-senior-dev instructions. It exposes
+the same ruleset the Claude hooks and Pi extension use, so every host emits
+identical rules.
+
+It is not a replacement for the always-on adapters. Ponytail normally lives in
+the system context every turn. MCP prompts are user-invoked, and there is no
+portable MCP primitive for "inject this into every turn" across hosts. So this
+server is the clean option for MCP hosts whose only injection point is the
+prompt menu, or that pull context through tools. See issue #70.
+
+## What it exposes
+
+- Prompt `ponytail` — returns the ruleset as a user message. Optional `mode`
+  argument: `lite`, `full`, or `ultra`. Omit it to use the configured default.
+- Tool `ponytail_instructions` — same text, plus `structuredContent`
+  (`{ mode, instructions }`), for hosts that pull context via tools or code
+  execution. Read-only.
+
+Mode resolution reuses `hooks/ponytail-config.js`, so `PONYTAIL_DEFAULT_MODE`
+and `~/.config/ponytail/config.json` work the same as everywhere else.
+
+## Run it
+
+```bash
+cd ponytail-mcp
+npm install
+node index.js        # speaks MCP over stdio
+```
+
+Point an MCP host at that command. Example client entry:
+
+```json
+{ "mcpServers": { "ponytail": { "command": "node", "args": ["ponytail-mcp/index.js"] } } }
+```
+
+## Test
+
+```bash
+npm test
+```
+
+Covers mode resolution and the instruction text. The MCP wiring in `index.js`
+is intentionally thin: it just maps the prompt and tool onto
+`buildInstructions`.