feat: add shared config module for project-local browse state

Centralizes path resolution (git root detection, state dir, log paths) into
config.ts. Both cli.ts and server.ts import from it, eliminating duplicated
PORT_OFFSET/BROWSE_PORT/STATE_FILE logic.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

browse/src/config.ts

@@ -0,0 +1,105 @@
+/**
+ * Shared config for browse CLI + server.
+ *
+ * Resolution:
+ *   1. BROWSE_STATE_FILE env → derive stateDir from parent
+ *   2. git rev-parse --show-toplevel → projectDir/.gstack/
+ *   3. process.cwd() fallback (non-git environments)
+ *
+ * The CLI computes the config and passes BROWSE_STATE_FILE to the
+ * spawned server. The server derives all paths from that env var.
+ */
+
+import * as fs from 'fs';
+import * as path from 'path';
+
+export interface BrowseConfig {
+  projectDir: string;
+  stateDir: string;
+  stateFile: string;
+  consoleLog: string;
+  networkLog: string;
+  dialogLog: string;
+}
+
+/**
+ * Detect the git repository root, or null if not in a repo / git unavailable.
+ */
+export function getGitRoot(): string | null {
+  try {
+    const proc = Bun.spawnSync(['git', 'rev-parse', '--show-toplevel'], {
+      stdout: 'pipe',
+      stderr: 'pipe',
+      timeout: 2_000, // Don't hang if .git is broken
+    });
+    if (proc.exitCode !== 0) return null;
+    return proc.stdout.toString().trim() || null;
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Resolve all browse config paths.
+ *
+ * If BROWSE_STATE_FILE is set (e.g. by CLI when spawning server, or by
+ * tests for isolation), all paths are derived from it. Otherwise, the
+ * project root is detected via git or cwd.
+ */
+export function resolveConfig(
+  env: Record<string, string | undefined> = process.env,
+): BrowseConfig {
+  let stateFile: string;
+  let stateDir: string;
+  let projectDir: string;
+
+  if (env.BROWSE_STATE_FILE) {
+    stateFile = env.BROWSE_STATE_FILE;
+    stateDir = path.dirname(stateFile);
+    projectDir = path.dirname(stateDir); // parent of .gstack/
+  } else {
+    projectDir = getGitRoot() || process.cwd();
+    stateDir = path.join(projectDir, '.gstack');
+    stateFile = path.join(stateDir, 'browse.json');
+  }
+
+  return {
+    projectDir,
+    stateDir,
+    stateFile,
+    consoleLog: path.join(stateDir, 'browse-console.log'),
+    networkLog: path.join(stateDir, 'browse-network.log'),
+    dialogLog: path.join(stateDir, 'browse-dialog.log'),
+  };
+}
+
+/**
+ * Create the .gstack/ state directory if it doesn't exist.
+ * Throws with a clear message on permission errors.
+ */
+export function ensureStateDir(config: BrowseConfig): void {
+  try {
+    fs.mkdirSync(config.stateDir, { recursive: true });
+  } catch (err: any) {
+    if (err.code === 'EACCES') {
+      throw new Error(`Cannot create state directory ${config.stateDir}: permission denied`);
+    }
+    if (err.code === 'ENOTDIR') {
+      throw new Error(`Cannot create state directory ${config.stateDir}: a file exists at that path`);
+    }
+    throw err;
+  }
+}
+
+/**
+ * Read the binary version (git SHA) from browse/dist/.version.
+ * Returns null if the file doesn't exist or can't be read.
+ */
+export function readVersionHash(execPath: string = process.execPath): string | null {
+  try {
+    const versionFile = path.resolve(path.dirname(execPath), '.version');
+    return fs.readFileSync(versionFile, 'utf-8').trim() || null;
+  } catch {
+    return null;
+  }
+}