feat: extract shared utilities into lib/util.ts

DRY up atomicWriteSync, readJSON, getGitInfo, getVersion, getRemoteSlug,
and sanitizeForFilename from eval-store.ts, session-runner.ts, and
eval-watch.ts into a shared module.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/util.ts

@@ -0,0 +1,126 @@
+/**
+ * Shared utilities for gstack.
+ *
+ * Extracted from eval-store.ts, session-runner.ts, eval-watch.ts to avoid
+ * duplication. All functions are pure or side-effect-minimal.
+ */
+
+import * as fs from 'fs';
+import * as path from 'path';
+import * as os from 'os';
+import { spawnSync } from 'child_process';
+
+// --- Paths ---
+
+export const GSTACK_STATE_DIR = process.env.GSTACK_STATE_DIR || path.join(os.homedir(), '.gstack');
+export const GSTACK_DEV_DIR = path.join(os.homedir(), '.gstack-dev');
+
+// --- File I/O ---
+
+/** Atomic write: write to .tmp then rename. Non-fatal on error. */
+export function atomicWriteSync(filePath: string, data: string): void {
+  const tmp = filePath + '.tmp';
+  fs.writeFileSync(tmp, data);
+  fs.renameSync(tmp, filePath);
+}
+
+/** Atomic JSON write: stringify + atomic write. Creates parent dirs. */
+export function atomicWriteJSON(filePath: string, data: unknown, mode?: number): void {
+  fs.mkdirSync(path.dirname(filePath), { recursive: true });
+  const content = JSON.stringify(data, null, 2) + '\n';
+  atomicWriteSync(filePath, content);
+  if (mode !== undefined) {
+    fs.chmodSync(filePath, mode);
+  }
+}
+
+/** Read and parse a JSON file, returning null on any error. */
+export function readJSON<T>(filePath: string): T | null {
+  try {
+    return JSON.parse(fs.readFileSync(filePath, 'utf-8'));
+  } catch {
+    return null;
+  }
+}
+
+// --- Git ---
+
+/** Detect the git repository root, or null if not in a repo. */
+export function getGitRoot(): string | null {
+  try {
+    const proc = spawnSync('git', ['rev-parse', '--show-toplevel'], {
+      stdio: 'pipe',
+      timeout: 2_000,
+    });
+    if (proc.status !== 0) return null;
+    return proc.stdout?.toString().trim() || null;
+  } catch {
+    return null;
+  }
+}
+
+/** Get current branch name and short SHA. */
+export function getGitInfo(): { branch: string; sha: string } {
+  try {
+    const branch = spawnSync('git', ['rev-parse', '--abbrev-ref', 'HEAD'], { stdio: 'pipe', timeout: 5000 });
+    const sha = spawnSync('git', ['rev-parse', '--short', 'HEAD'], { stdio: 'pipe', timeout: 5000 });
+    return {
+      branch: branch.stdout?.toString().trim() || 'unknown',
+      sha: sha.stdout?.toString().trim() || 'unknown',
+    };
+  } catch {
+    return { branch: 'unknown', sha: 'unknown' };
+  }
+}
+
+/**
+ * Derive a slug from the git remote origin URL (owner-repo format).
+ * Falls back to the directory basename if no remote is configured.
+ */
+export function getRemoteSlug(): string {
+  try {
+    const proc = spawnSync('git', ['remote', 'get-url', 'origin'], {
+      stdio: 'pipe',
+      timeout: 2_000,
+    });
+    if (proc.status !== 0) throw new Error('no remote');
+    const url = proc.stdout?.toString().trim() || '';
+    // SSH:   git@github.com:owner/repo.git → owner-repo
+    // HTTPS: https://github.com/owner/repo.git → owner-repo
+    const match = url.match(/[:/]([^/]+)\/([^/]+?)(?:\.git)?$/);
+    if (match) return `${match[1]}-${match[2]}`;
+    throw new Error('unparseable');
+  } catch {
+    const root = getGitRoot();
+    return path.basename(root || process.cwd());
+  }
+}
+
+// --- Version ---
+
+/** Read the gstack version from package.json. */
+export function getVersion(): string {
+  try {
+    // Try relative to this file first (lib/), then try common locations
+    const candidates = [
+      path.resolve(__dirname, '..', 'package.json'),
+      path.resolve(__dirname, '..', '..', 'package.json'),
+    ];
+    for (const pkgPath of candidates) {
+      try {
+        const pkg = JSON.parse(fs.readFileSync(pkgPath, 'utf-8'));
+        if (pkg.version) return pkg.version;
+      } catch { continue; }
+    }
+    return 'unknown';
+  } catch {
+    return 'unknown';
+  }
+}
+
+// --- String helpers ---
+
+/** Sanitize a name for use as a filename: strip leading slashes, replace / with - */
+export function sanitizeForFilename(name: string): string {
+  return name.replace(/^\/+/, '').replace(/\//g, '-');
+}