fix: apply the review items for #755 at merge

Say that settings.yaml is shared only until the session's first settings write
(dsh saves by rename, which replaces the link), warn when a session's own DSH_HOME
does not answer and the server's home is linked instead, test
guardSessionConfigDirHome, and document the linkFromRealHome exception and what a
custom-model dsh session does not carry over.

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

test/custom-model-session-home-guard.test.ts

@@ -0,0 +1,68 @@
+/**
+ * @fileoverview `guardSessionConfigDirHome` probes a session-supplied configDir home before
+ * the apply runs synchronous fs calls on it (the #516 rule): a home that does not answer is
+ * hidden from the apply, so it links from the server's own home instead and says so.
+ *
+ * The probe is mocked: a mount that never answers is the case under test, and a real one
+ * cannot be made here.
+ */
+import { afterEach, describe, expect, it, vi } from 'vitest';
+
+vi.mock('../src/utils/bounded-path-probe.js', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('../src/utils/bounded-path-probe.js')>();
+  return { ...actual, probePathKind: vi.fn() };
+});
+
+import { getCli } from '../src/config/cli-registry/registry.js';
+import { probePathKind } from '../src/utils/bounded-path-probe.js';
+import { guardSessionConfigDirHome } from '../src/web/routes/session-routes.js';
+
+const probe = vi.mocked(probePathKind);
+const deepseek = () => getCli('deepseek')!;
+const env =
+  (values: Record<string, string>) =>
+  (name: string): string | undefined =>
+    values[name];
+
+describe('guardSessionConfigDirHome', () => {
+  afterEach(() => {
+    probe.mockReset();
+    vi.restoreAllMocks();
+  });
+
+  it('hides only the home when it does not answer, and warns that the server home is used', async () => {
+    probe.mockResolvedValue('unknown');
+    const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+
+    const lookup = await guardSessionConfigDirHome(deepseek(), env({ DSH_HOME: '/mnt/dead/dsh', OTHER: 'kept' }));
+
+    expect(lookup('DSH_HOME')).toBeUndefined();
+    expect(lookup('OTHER')).toBe('kept');
+    expect(probe).toHaveBeenCalledWith('/mnt/dead/dsh', { pastCap: true });
+    expect(warn).toHaveBeenCalledTimes(1);
+    expect(String(warn.mock.calls[0][0])).toContain('/mnt/dead/dsh');
+  });
+
+  it('passes a home that answers straight through', async () => {
+    probe.mockResolvedValue('directory');
+    const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+    const base = env({ DSH_HOME: '/home/u/.dsh' });
+
+    expect(await guardSessionConfigDirHome(deepseek(), base)).toBe(base);
+    expect(warn).not.toHaveBeenCalled();
+  });
+
+  it('never probes a relative home: the apply refuses to join one', async () => {
+    const base = env({ DSH_HOME: 'relative/dsh' });
+
+    expect(await guardSessionConfigDirHome(deepseek(), base)).toBe(base);
+    expect(probe).not.toHaveBeenCalled();
+  });
+
+  it('never probes for a CLI with no linkFromRealHome recipe', async () => {
+    const base = env({ CODEX_HOME: '/mnt/dead/codex' });
+
+    expect(await guardSessionConfigDirHome(getCli('codex')!, base)).toBe(base);
+    expect(probe).not.toHaveBeenCalled();
+  });
+});