refactor: move init-authoring helpers into the backend

The scaffold + repo-detection + headless-Claude-run helpers that `webmux init`
uses (buildStarterTemplate, detectInitProjectContext, buildInitPromptSpec,
buildInitAgentCommand, runInitAgentCommand, stream parsing) move from
bin/src/init-helpers.ts to backend/src/services/init-authoring.ts so the
server can drive the same flow for an upcoming "set up project on add"
feature. `bin/src/init.ts` now imports them from the backend.

Generic process/git/repo primitives (run, which, getGitRoot, detectProjectName,
RunResult) move to backend/src/lib/shell.ts and are re-exported from
bin/src/shared.ts, so every existing `./shared` consumer is unchanged and the
backend can share them without a backend→bin import. Test moves alongside the
code into backend/src/__tests__.

No behavior change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hugocasa

bin/src/init-helpers.test.ts …end/src/__tests__/init-authoring.test.tsbin/src/init-helpers.test.ts renamed to backend/src/__tests__/init-authoring.test.ts bin/src/init-helpers.test.ts renamed to backend/src/__tests__/init-authoring.test.ts

@@ -2,14 +2,14 @@ import { afterEach, describe, expect, it } from "bun:test";
 import { mkdtemp, rm } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
-import { loadConfig } from "../../backend/src/adapters/config.ts";
+import { loadConfig } from "../adapters/config";
 import {
   buildInitAgentCommand,
   buildInitPromptSpec,
   buildStarterTemplate,
   detectInitProjectContext,
   parseInitAgentStreamLine,
-} from "./init-helpers.ts";
+} from "../services/init-authoring";
 
 describe("buildInitAgentCommand", () => {
   const prompt = {

backend/src/lib/shell.ts

@@ -0,0 +1,41 @@
+import { existsSync, readFileSync } from "node:fs";
+import { basename, join } from "node:path";
+
+export interface RunResult {
+  success: boolean;
+  stdout: Buffer;
+  stderr: Buffer;
+}
+
+/** Run a command synchronously and capture its output. Shared by the CLI
+ *  (re-exported from bin/src/shared.ts) and backend code that needs to shell
+ *  out (e.g. repo detection for project setup). */
+export function run(cmd: string, args: string[], opts?: { cwd?: string }): RunResult {
+  const result = Bun.spawnSync([cmd, ...args], { stdout: "pipe", stderr: "pipe", ...opts });
+  return {
+    success: result.success,
+    stdout: result.stdout as Buffer,
+    stderr: result.stderr as Buffer,
+  };
+}
+
+export function which(tool: string): boolean {
+  return run("which", [tool]).success;
+}
+
+export function getGitRoot(): string | null {
+  const result = run("git", ["rev-parse", "--show-toplevel"]);
+  if (!result.success) return null;
+  return result.stdout.toString().trim();
+}
+
+export function detectProjectName(gitRoot: string): string {
+  const pkgPath = join(gitRoot, "package.json");
+  if (existsSync(pkgPath)) {
+    try {
+      const pkg = JSON.parse(readFileSync(pkgPath, "utf-8"));
+      if (pkg.name) return pkg.name;
+    } catch {} // malformed package.json, fall back to dir name
+  }
+  return basename(gitRoot);
+}

bin/src/init-helpers.ts backend/src/services/init-authoring.tsbin/src/init-helpers.ts renamed to backend/src/services/init-authoring.ts

@@ -1,7 +1,7 @@
 import { existsSync, readFileSync, rmSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
-import { detectProjectName, run } from "./shared.ts";
+import { detectProjectName, run } from "../lib/shell";
 
 export type InitAuthoringChoice = "claude" | "codex" | "manual";
 export type InitAgent = Exclude<InitAuthoringChoice, "manual">;

bin/src/init.ts

@@ -13,7 +13,7 @@ import {
   type InitAgent,
   type InitAgentStreamEvent,
   type InitAuthoringChoice,
-} from "./init-helpers.ts";
+} from "../../backend/src/services/init-authoring.ts";
 
 // ── Dependency checks ───────────────────────────────────────────────────────
 

bin/src/shared.ts

@@ -1,31 +1,12 @@
-import { existsSync, readFileSync, realpathSync } from "node:fs";
-import { basename, dirname, join, resolve } from "node:path";
+import { dirname, resolve } from "node:path";
+import { realpathSync } from "node:fs";
 import { createApi } from "@webmux/api-contract";
+import { run } from "../../backend/src/lib/shell";
 
-export interface RunResult {
-  success: boolean;
-  stdout: Buffer;
-  stderr: Buffer;
-}
-
-export function run(cmd: string, args: string[], opts?: { cwd?: string }): RunResult {
-  const result = Bun.spawnSync([cmd, ...args], { stdout: "pipe", stderr: "pipe", ...opts });
-  return {
-    success: result.success,
-    stdout: result.stdout as Buffer,
-    stderr: result.stderr as Buffer,
-  };
-}
-
-export function which(tool: string): boolean {
-  return run("which", [tool]).success;
-}
-
-export function getGitRoot(): string | null {
-  const result = run("git", ["rev-parse", "--show-toplevel"]);
-  if (!result.success) return null;
-  return result.stdout.toString().trim();
-}
+// Generic process/git/repo primitives live in the backend lib so backend code
+// (e.g. project setup) can share them; re-export here so existing CLI imports
+// from "./shared" keep working.
+export { run, which, getGitRoot, detectProjectName, type RunResult } from "../../backend/src/lib/shell";
 
 /**
  * Thrown by argparse functions to signal usage errors (e.g. missing flag value,
@@ -34,17 +15,6 @@ export function getGitRoot(): string | null {
  */
 export class CommandUsageError extends Error {}
 
-export function detectProjectName(gitRoot: string): string {
-  const pkgPath = join(gitRoot, "package.json");
-  if (existsSync(pkgPath)) {
-    try {
-      const pkg = JSON.parse(readFileSync(pkgPath, "utf-8"));
-      if (pkg.name) return pkg.name;
-    } catch {} // malformed package.json, fall back to dir name
-  }
-  return basename(gitRoot);
-}
-
 /**
  * When the webmux server isn't reachable the bare error message is unhelpful to
  * users. Older Bun threw a `TypeError: fetch failed`; current Bun throws