feat(cases): create a new case in a custom folder

POST /api/cases takes an optional path; Add Case > Create New gets a 'Create in a
custom folder' option with Browse. The folder is created (or an empty one filled),
scaffolded like a normal case and registered as a linked case. System, home,
credential and Codeman folders are refused; a folder with files is Link Existing's
job; a failure after the first write undoes what this call created. Admin only in
multi-user mode, like Link Existing.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/web/case-path.ts

@@ -0,0 +1,137 @@
+/**
+ * @fileoverview Validation for "create a new case in a custom folder" (`POST /api/cases` with a
+ * `path`). Creating a case writes a scaffold (`CLAUDE.md`, `src/`, `.claude/settings.local.json`)
+ * and registers the folder in the shared, ownerless linked-cases registry, so the target has to be
+ * judged before anything is created:
+ *
+ *   - it must be an absolute path (a leading `~` is expanded) with no traversal and none of the shell
+ *     metacharacters a session's working directory is later rejected for (`isValidWorkingDir`), so
+ *     a case this accepts is one a session can actually start in;
+ *   - it must not be a system directory, the home directory itself, Codeman's own data directory, or
+ *     a credential/config tree (`~/.ssh`, `~/.aws`, `~/.claude`, ...). Judged on the path as typed AND on
+ *     its symlink-resolved form, so a link into `/etc` is not a way around it;
+ *   - its parent must already exist (one folder is created, never a whole chain), and the folder
+ *     itself must not exist or must be an EMPTY directory (a folder with contents is Link Existing's
+ *     job, and silently scaffolding into someone's project is the one thing this must never do);
+ *   - it must not be a symlink.
+ *
+ * Pure except for the filesystem reads in `prepareNewCasePath`; the policy lives in `blockedReason`
+ * so it can be tested without a disk.
+ *
+ * @module web/case-path
+ */
+
+import { promises as fs } from 'node:fs';
+import { basename, dirname, join, resolve, sep } from 'node:path';
+import { isValidWorkingDir } from './schemas.js';
+
+/** System trees nobody creates a project in; creating one here is a mistake or an attack. */
+const BLOCKED_SYSTEM_ROOTS = [
+  '/bin',
+  '/boot',
+  '/dev',
+  '/etc',
+  '/lib',
+  '/lib32',
+  '/lib64',
+  '/proc',
+  '/run',
+  '/sbin',
+  '/sys',
+  '/usr',
+];
+
+/** Home-relative trees that hold credentials or other tools' own configuration. */
+const BLOCKED_HOME_DIRS = ['.ssh', '.gnupg', '.aws', '.kube', '.docker', '.claude', '.codex', '.gemini'];
+
+export interface NewCasePathContext {
+  home: string;
+  /** Codeman's own state directory (`getDataDir()`), which must never become a case. */
+  dataDir: string;
+}
+
+export type NewCasePathResult =
+  | { ok: true; path: string; existedEmpty: boolean }
+  | { ok: false; code: 'INVALID' | 'BLOCKED' | 'NOT_FOUND' | 'EXISTS'; reason: string };
+
+const isWithin = (child: string, root: string): boolean =>
+  child === root || child.startsWith(root.endsWith(sep) ? root : root + sep);
+
+/** `~` and `~/x` to the home directory; anything else is returned unchanged. */
+export function expandHome(raw: string, home: string): string {
+  if (raw === '~') return home;
+  if (raw.startsWith('~/')) return join(home, raw.slice(2));
+  return raw;
+}
+
+/** Why a case may not live at this (already absolute and normalised) path, or null. */
+export function blockedReason(absPath: string, ctx: NewCasePathContext): string | null {
+  if (absPath === sep) return 'The filesystem root cannot be a case';
+  for (const root of BLOCKED_SYSTEM_ROOTS) {
+    if (isWithin(absPath, root)) return `${root} is a system directory`;
+  }
+  if (absPath === ctx.home) return 'The home folder itself cannot be a case; pick a folder inside it';
+  for (const dir of BLOCKED_HOME_DIRS) {
+    if (isWithin(absPath, join(ctx.home, dir))) return `~/${dir} holds credentials or another tool's configuration`;
+  }
+  if (isWithin(absPath, ctx.dataDir)) return "Codeman's own data folder cannot be a case";
+  // Any Codeman instance's data dir under the home folder (~/.codeman, ~/.codeman-beta, ...), not only
+  // the one this process uses.
+  if (absPath.startsWith(ctx.home + sep)) {
+    const firstSegment = absPath.slice(ctx.home.length + 1).split(sep)[0];
+    if (/^\.codeman/.test(firstSegment)) return "Codeman's own data folder cannot be a case";
+  }
+  return null;
+}
+
+/**
+ * Judge `raw` as the folder for a new case and, if it is acceptable, say what to create.
+ * Never creates anything.
+ */
+export async function prepareNewCasePath(raw: string, ctx: NewCasePathContext): Promise<NewCasePathResult> {
+  const typed = raw.trim();
+  if (!typed) return { ok: false, code: 'INVALID', reason: 'Enter a folder path' };
+  const expanded = expandHome(typed, ctx.home);
+  if (!isValidWorkingDir(expanded)) {
+    return {
+      ok: false,
+      code: 'INVALID',
+      reason: 'Use an absolute path with letters, numbers, spaces, - _ . only (no .., no shell characters)',
+    };
+  }
+
+  const target = resolve(expanded);
+  const typedBlock = blockedReason(target, ctx);
+  if (typedBlock) return { ok: false, code: 'BLOCKED', reason: typedBlock };
+
+  // Resolve the parent's symlinks, then judge again: a link into a blocked tree must not pass.
+  let realParent: string;
+  try {
+    realParent = await fs.realpath(dirname(target));
+    if (!(await fs.stat(realParent)).isDirectory()) {
+      return { ok: false, code: 'INVALID', reason: `${dirname(target)} is not a folder` };
+    }
+  } catch {
+    return { ok: false, code: 'NOT_FOUND', reason: `The parent folder ${dirname(target)} does not exist` };
+  }
+  const real = join(realParent, basename(target));
+  const realBlock = blockedReason(real, ctx);
+  if (realBlock) return { ok: false, code: 'BLOCKED', reason: realBlock };
+
+  try {
+    const st = await fs.lstat(real);
+    if (st.isSymbolicLink()) return { ok: false, code: 'INVALID', reason: `${target} is a symbolic link` };
+    if (!st.isDirectory()) return { ok: false, code: 'INVALID', reason: `${target} exists and is not a folder` };
+    if ((await fs.readdir(real)).length > 0) {
+      return {
+        ok: false,
+        code: 'EXISTS',
+        reason: `${target} already has files in it. Use "Link Existing" for a project that already exists`,
+      };
+    }
+    return { ok: true, path: real, existedEmpty: true };
+  } catch (err) {
+    if ((err as NodeJS.ErrnoException).code === 'ENOENT') return { ok: true, path: real, existedEmpty: false };
+    return { ok: false, code: 'INVALID', reason: `Cannot read ${target}: ${(err as Error).message}` };
+  }
+}