fix(test): guard the CASES_DIR delete + harden the data-dir teardown

PR #356 stopped the remote-hosts.json fixture write from clobbering prod.
Two holes in the same file remain:

1. The quick-start afterEach still ran rmSync(CASES_DIR, recursive).
   CASES_DIR is join(homedir(), 'codeman-cases'), and on Linux builds
   where os.homedir() reads /etc/passwd instead of $HOME it resolves to
   the PROD case tree - so a full-suite run deleted the real
   ~/codeman-cases. Add a shared safeRmHomeTree() containment gate that
   only deletes a path under the redirected test HOME.

2. setup.ts teardown did rmSync(process.env.CODEMAN_DATA_DIR ?? '') AFTER
   restoring the env - if a pre-existing prod CODEMAN_DATA_DIR was set,
   that deleted prod. Capture the throwaway dir in a const and clean that.

A broader test-isolation sweep (10 files: cli-skill-target, edge-cases,
integration-flows, operation-lightspeed, ralph-integration,
case-clone-routes, voice-routes, session-cleanup, sse-events,
sse-subscription-filter) also applies the same containment gates to every
per-case delete. It is intentionally NOT included here to keep this PR
skinny; it is identified and available on request.

test/mocks/index.ts

@@ -7,5 +7,5 @@
 
 export { MockSession, createMockSession, terminalOutputs } from './mock-session.js';
 export { MockStateStore } from './mock-state-store.js';
-export { waitForEvent, createDeferred } from './test-helpers.js';
+export { waitForEvent, createDeferred, safeRmHomeTree, isUnderTestHome } from './test-helpers.js';
 export { createMockRouteContext, type MockRouteContext } from './mock-route-context.js';

test/mocks/test-helpers.ts

@@ -2,6 +2,9 @@
  * Reusable async test helpers.
  */
 
+import { rmSync } from 'node:fs';
+import { resolve } from 'node:path';
+
 /** Wait for an EventEmitter to emit a specific event, with timeout */
 export function waitForEvent(
   emitter: { once: (event: string, listener: (...args: unknown[]) => void) => void },
@@ -34,3 +37,39 @@ export function createDeferred<T = void>(): {
   });
   return { promise, resolve, reject };
 }
+
+/**
+ * Delete a directory tree, but ONLY when it lives inside the test HOME.
+ *
+ * SAFETY (2026-08-29): `test/setup.ts` redirects `process.env.HOME` to a
+ * throwaway dir, but code that resolves paths via `os.homedir()` does NOT
+ * follow that redirect on every Linux build/Node version — some read
+ * /etc/passwd instead of $HOME. A test that `rmSync(CASES_DIR, recursive)` can
+ * therefore delete the PRODUCTION `~/codeman-cases` (or any home-anchored
+ * tree) on those platforms. This gate refuses to delete anything not under the
+ * (redirected) `process.env.HOME`. Lexical `resolve()` is used because the leaf
+ * often does not exist and `realpathSync` would throw.
+ */
+export function safeRmHomeTree(path: string): void {
+  const home = process.env.HOME;
+  if (!home) return;
+  const target = resolve(path);
+  const root = resolve(home);
+  if (target === root || target.startsWith(root + '/')) {
+    rmSync(target, { recursive: true, force: true });
+  }
+}
+
+/**
+ * True when `path` resolves strictly inside `process.env.HOME` (or to it).
+ * Same rationale as `safeRmHomeTree`; use for guarded non-recursive deletes
+ * (single files like `linked-cases.json`) so they never touch prod state on
+ * platforms where `os.homedir()` ignores `$HOME`.
+ */
+export function isUnderTestHome(path: string): boolean {
+  const home = process.env.HOME;
+  if (!home) return false;
+  const target = resolve(path);
+  const root = resolve(home);
+  return target === root || target.startsWith(root + '/');
+}