feat(fleet): harden v1 migration preview

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/mosaic/src/fleet/generated-env-boundary.spec.ts

@@ -9,7 +9,7 @@ import {
   symlink,
   writeFile,
 } from 'node:fs/promises';
-import { tmpdir } from 'node:os';
+import { homedir, tmpdir } from 'node:os';
 import { join } from 'node:path';
 import { afterEach, describe, expect, it } from 'vitest';
 import {
@@ -87,6 +87,42 @@ describe('generated fleet agent environment boundary', (): void => {
     }).toThrow(AgentEnvBoundaryError);
   });
 
+  it('rejects traversal in home-relative workdirs before expansion', (): void => {
+    for (const workingDirectory of ['~/../escape', '~/src/../../escape']) {
+      expect((): void => {
+        renderGeneratedAgentEnvironment({
+          ...generatedValues,
+          MOSAIC_AGENT_WORKDIR: workingDirectory,
+        });
+      }).toThrow(
+        expect.objectContaining({
+          diagnostic: expect.objectContaining({
+            code: 'unsafe-path',
+            key: 'MOSAIC_AGENT_WORKDIR',
+          }),
+        }),
+      );
+    }
+  });
+
+  it('expands home-relative workdirs before preserving absolute-path validation', (): void => {
+    expect(
+      renderGeneratedAgentEnvironment({
+        ...generatedValues,
+        MOSAIC_AGENT_WORKDIR: '~/src',
+      }),
+    ).toContain(`MOSAIC_AGENT_WORKDIR=${join(homedir(), 'src')}\n`);
+
+    for (const workingDirectory of ['relative/path', '../outside']) {
+      expect((): void => {
+        renderGeneratedAgentEnvironment({
+          ...generatedValues,
+          MOSAIC_AGENT_WORKDIR: workingDirectory,
+        });
+      }).toThrow(AgentEnvBoundaryError);
+    }
+  });
+
   it('previews legacy relocation and quarantine without exposing content or mutating files', async (): Promise<void> => {
     cleanup = await mkdtemp(join(tmpdir(), 'mosaic-generated-env-'));
     const mosaicHome = join(cleanup, 'mosaic');