feat: add mex pattern add command

src/cli.ts

@@ -85,6 +85,25 @@ program
     }
   });
 
+// ── Layer 4: Patterns ──
+const patternCmd = program
+  .command("pattern")
+  .description("Manage pattern files");
+
+patternCmd
+  .command("add <name>")
+  .description("Create a new pattern file and add it to the index")
+  .action(async (name) => {
+    try {
+      const config = findConfig();
+      const { runPatternAdd } = await import("./pattern/index.js");
+      await runPatternAdd(config, name);
+    } catch (err) {
+      console.error((err as Error).message);
+      process.exit(1);
+    }
+  });
+
 // ── Git Hook ──
 program
   .command("watch")
@@ -116,6 +135,7 @@ program
     console.log("  mex sync --warnings    Include warning-only files in sync");
     console.log("  mex init               Pre-scan codebase, build brief for AI");
     console.log("  mex init --json        Scanner brief as JSON");
+    console.log("  mex pattern add <name> Create a new pattern file");
     console.log("  mex watch              Install post-commit hook for auto drift score");
     console.log("  mex watch --uninstall  Remove the post-commit hook");
     console.log();

src/pattern/index.ts

@@ -0,0 +1,68 @@
+import { join } from "node:path";
+import { existsSync, writeFileSync, appendFileSync, mkdirSync, readFileSync } from "node:fs";
+import chalk from "chalk";
+import type { MexConfig } from "../types.js";
+
+export async function runPatternAdd(config: MexConfig, name: string) {
+  if (!/^[a-z0-9-]+$/i.test(name)) {
+    throw new Error(`Invalid pattern name '${name}'. Use only letters, numbers, and hyphens.`);
+  }
+
+  const patternsDir = join(config.scaffoldRoot, "patterns");
+  const patternPath = join(patternsDir, `${name}.md`);
+  const indexPath = join(patternsDir, "INDEX.md");
+
+  if (existsSync(patternPath)) {
+    throw new Error(`Pattern '${name}' already exists at ${patternPath}`);
+  }
+
+  const today = new Date().toISOString().split("T")[0];
+
+  const template = `---
+name: ${name}
+description: [one line — what this pattern covers and when to use it]
+triggers:
+  - "[keyword that should trigger loading this file]"
+edges:
+  - target: "context/conventions.md"
+    condition: "when verifying this task"
+last_updated: ${today}
+---
+
+# ${name}
+
+## Context
+[What to load or know before starting this task type]
+
+## Steps
+[The workflow — what to do, in what order]
+
+## Gotchas
+[The things that go wrong. What to watch out for.]
+
+## Verify
+[Checklist to run after completing this task type]
+
+## Debug
+[What to check when this task type breaks]
+
+## Update Scaffold
+- [ ] Update \`ROUTER.md\` "Current Project State" if what's working/not built has changed
+- [ ] Update any \`context/\` files that are now out of date
+- [ ] If this is a new task type without a pattern, create one in \`patterns/\` and add to \`INDEX.md\`
+`;
+
+  mkdirSync(patternsDir, { recursive: true });
+  writeFileSync(patternPath, template, "utf8");
+
+  if (existsSync(indexPath)) {
+    const currentIndex = readFileSync(indexPath, "utf8");
+    const newlinePrefix = currentIndex.length === 0 || currentIndex.endsWith("\n") ? "" : "\n";
+    const entry = `${newlinePrefix}| [${name}.md](${name}.md) | [description] |\n`;
+    appendFileSync(indexPath, entry, "utf8");
+  }
+
+  console.log(chalk.green(`✓ Created pattern ${name}.md`));
+  console.log(chalk.dim(`  Added entry to patterns/INDEX.md`));
+  console.log(chalk.yellow(`! Remember to edit patterns/INDEX.md and replace [description] with a real use case.`));
+}

test/pattern.test.ts

@@ -0,0 +1,66 @@
+import { describe, it, expect, beforeEach, afterEach } from "vitest";
+import { mkdtempSync, mkdirSync, readFileSync, writeFileSync, rmSync } from "node:fs";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+import { runPatternAdd } from "../src/pattern/index.js";
+
+let tmpDir: string;
+
+beforeEach(() => {
+  tmpDir = mkdtempSync(join(tmpdir(), "mex-pattern-"));
+  mkdirSync(join(tmpDir, "patterns"));
+});
+
+afterEach(() => {
+  rmSync(tmpDir, { recursive: true, force: true });
+});
+
+describe("runPatternAdd", () => {
+  it("creates a new pattern file and index entry", async () => {
+    writeFileSync(join(tmpDir, "patterns", "INDEX.md"), "| Pattern | Use when |\n|---|---|\n", "utf8");
+
+    await runPatternAdd({ projectRoot: tmpDir, scaffoldRoot: tmpDir }, "my-pattern");
+
+    const patternContent = readFileSync(join(tmpDir, "patterns", "my-pattern.md"), "utf8");
+    expect(patternContent).toContain("name: my-pattern");
+    expect(patternContent).toContain("# my-pattern");
+    expect(patternContent).toContain("## Verify");
+
+    const indexContent = readFileSync(join(tmpDir, "patterns", "INDEX.md"), "utf8");
+    expect(indexContent).toContain("| [my-pattern.md](my-pattern.md) |");
+  });
+
+  it("throws an error if pattern already exists", async () => {
+    writeFileSync(join(tmpDir, "patterns", "my-pattern.md"), "existing content", "utf8");
+
+    await expect(
+      runPatternAdd({ projectRoot: tmpDir, scaffoldRoot: tmpDir }, "my-pattern")
+    ).rejects.toThrow("already exists");
+  });
+
+  it("creates pattern even if INDEX.md is missing", async () => {
+    await runPatternAdd({ projectRoot: tmpDir, scaffoldRoot: tmpDir }, "my-pattern");
+
+    const patternContent = readFileSync(join(tmpDir, "patterns", "my-pattern.md"), "utf8");
+    expect(patternContent).toContain("name: my-pattern");
+  });
+
+  it("throws an error for invalid pattern names", async () => {
+    await expect(
+      runPatternAdd({ projectRoot: tmpDir, scaffoldRoot: tmpDir }, "my pattern")
+    ).rejects.toThrow("Invalid pattern name");
+
+    await expect(
+      runPatternAdd({ projectRoot: tmpDir, scaffoldRoot: tmpDir }, "pattern!")
+    ).rejects.toThrow("Invalid pattern name");
+  });
+
+  it("appends to INDEX.md with a newline if it does not end with one", async () => {
+    writeFileSync(join(tmpDir, "patterns", "INDEX.md"), "| Pattern | Use when |", "utf8");
+
+    await runPatternAdd({ projectRoot: tmpDir, scaffoldRoot: tmpDir }, "newline-pattern");
+
+    const indexContent = readFileSync(join(tmpDir, "patterns", "INDEX.md"), "utf8");
+    expect(indexContent).toBe("| Pattern | Use when |\n| [newline-pattern.md](newline-pattern.md) | [description] |\n");
+  });
+});