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/test/instructions.test.js

@@ -0,0 +1,22 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { MODES, resolveMode, buildInstructions } from "../instructions.js";
+
+test("resolveMode keeps valid intensities", () => {
+  for (const mode of MODES) assert.equal(resolveMode(mode), mode);
+});
+
+test("resolveMode falls back to a runtime intensity for off/unknown/empty", () => {
+  // PONYTAIL_DEFAULT_MODE could be anything in CI, so just assert the contract:
+  // never returns "off", "review", or junk — always one of the served modes.
+  for (const input of ["off", "review", "nonsense", "", undefined, null]) {
+    assert.ok(MODES.includes(resolveMode(input)), `resolveMode(${input}) must be a served mode`);
+  }
+});
+
+test("buildInstructions returns the ruleset tagged with the resolved mode", () => {
+  const text = buildInstructions("ultra");
+  assert.match(text, /PONYTAIL MODE ACTIVE/);
+  assert.match(text, /ultra/);
+});