feat(fleet): brain-home split — fleet state under ~/.mosaic, templates stay config-home (#1298)

Co-authored-by: Jason Woltje <[email protected]>

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

@@ -176,6 +176,52 @@ describe('generated fleet agent environment boundary', (): void => {
     expect((await stat(result.generatedPath)).mode & 0o777).toBe(0o600);
   });
 
+  it('brain home: accepts and writes projections under MOSAIC_BRAIN_HOME/fleet/agents', async (): Promise<void> => {
+    const savedBrainHome = process.env['MOSAIC_BRAIN_HOME'];
+    try {
+      cleanup = await mkdtemp(join(tmpdir(), 'mosaic-generated-env-'));
+      const mosaicHome = join(cleanup, 'config-home');
+      const brainHome = join(cleanup, 'brain');
+      const agentEnvDir = join(brainHome, 'fleet', 'agents');
+      process.env['MOSAIC_BRAIN_HOME'] = brainHome;
+
+      const result = await writeAgentEnvironmentProjection({
+        mosaicHome,
+        agentEnvDir,
+        agentName: 'coder0',
+        generated: generatedValues,
+      });
+
+      // Projection landed in the brain tree, not under the config home.
+      expect(result.generatedPath).toBe(join(agentEnvDir, 'coder0.env.generated'));
+      expect((await stat(join(brainHome, 'fleet'))).mode & 0o777).toBe(0o700);
+      expect((await stat(agentEnvDir)).mode & 0o777).toBe(0o700);
+      expect((await stat(result.generatedPath)).mode & 0o777).toBe(0o600);
+      await expect(stat(join(mosaicHome, 'fleet'))).rejects.toThrow();
+
+      // A config-home agentEnvDir is now REJECTED while the brain is active —
+      // the boundary must not silently split state across two trees.
+      let rejected: unknown;
+      try {
+        await writeAgentEnvironmentProjection({
+          mosaicHome,
+          agentEnvDir: join(mosaicHome, 'fleet', 'agents'),
+          agentName: 'coder1',
+          generated: { ...generatedValues, MOSAIC_AGENT_NAME: 'coder1' },
+        });
+      } catch (caught: unknown) {
+        rejected = caught;
+      }
+      expect(rejected).toBeInstanceOf(AgentEnvBoundaryError);
+    } finally {
+      if (savedBrainHome === undefined) {
+        delete process.env['MOSAIC_BRAIN_HOME'];
+      } else {
+        process.env['MOSAIC_BRAIN_HOME'] = savedBrainHome;
+      }
+    }
+  });
+
   it('regenerates desired keys, relocates safe legacy local data, and quarantines forbidden legacy input', async (): Promise<void> => {
     cleanup = await mkdtemp(join(tmpdir(), 'mosaic-generated-env-'));
     const mosaicHome = join(cleanup, 'mosaic');