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/instructions.js

@@ -0,0 +1,26 @@
+// Pure instruction selection for the Ponytail MCP server. No MCP/SDK imports,
+// so this stays unit-testable on its own. Reuses the same builder the Claude
+// hooks and Pi extension use, so every host emits identical rules.
+import { createRequire } from "node:module";
+
+const require = createRequire(import.meta.url);
+const { getPonytailInstructions } = require("../hooks/ponytail-instructions.js");
+const { getDefaultMode, normalizeMode } = require("../hooks/ponytail-config.js");
+
+// The three intensities the server offers. "off" has no instructions to serve.
+export const MODES = ["lite", "full", "ultra"];
+
+// Resolve a requested mode to a runtime intensity. Unknown, empty, or "off"
+// falls back to the configured default, then to "full".
+// ponytail: keep the surface to these three; "off"/"review" aren't served here.
+export function resolveMode(requested) {
+  const asked = normalizeMode(requested);
+  if (asked && asked !== "off") return asked;
+
+  const fallback = normalizeMode(getDefaultMode());
+  return fallback && fallback !== "off" ? fallback : "full";
+}
+
+export function buildInstructions(requested) {
+  return getPonytailInstructions(resolveMode(requested));
+}