fix: make mosaic init idempotent — detect existing config files

- mosaic-init bash script: detect existing SOUL.md/USER.md/TOOLS.md and
  prompt user to keep, import (re-use values as defaults), or overwrite.
  Non-interactive mode exits cleanly unless --force is passed.
  Overwrite creates timestamped backups before replacing files.

- launch.ts checkSoul(): prefer 'mosaic wizard' over legacy bash script
  when SOUL.md is missing, with fallback to mosaic-init.

- detect-install.ts: pre-populate wizard state with existing values when
  user chooses 'reconfigure', so they see current settings as defaults.

- soul-setup.ts: show existing agent name and communication style as
  defaults during reconfiguration.

- Added tests for reconfigure pre-population and reset non-population.

packages/mosaic/__tests__/stages/detect-install.test.ts

@@ -65,4 +65,36 @@ describe('detectInstallStage', () => {
     expect(state.installAction).toBe('keep');
     expect(state.soul.agentName).toBe('TestAgent');
   });
+
+  it('pre-populates state when reconfiguring', async () => {
+    mkdirSync(join(tmpDir, 'bin'), { recursive: true });
+    writeFileSync(join(tmpDir, 'SOUL.md'), 'You are **Jarvis** in this session.');
+    writeFileSync(join(tmpDir, 'USER.md'), '**Name:** TestUser');
+
+    const p = new HeadlessPrompter({
+      'What would you like to do?': 'reconfigure',
+    });
+    const state = createState(tmpDir);
+    await detectInstallStage(p, state, mockConfig);
+
+    expect(state.installAction).toBe('reconfigure');
+    // Existing values loaded as defaults for reconfiguration
+    expect(state.soul.agentName).toBe('TestAgent');
+    expect(state.user.userName).toBe('TestUser');
+  });
+
+  it('does not pre-populate state on fresh reset', async () => {
+    mkdirSync(join(tmpDir, 'bin'), { recursive: true });
+    writeFileSync(join(tmpDir, 'SOUL.md'), 'You are **Jarvis** in this session.');
+
+    const p = new HeadlessPrompter({
+      'What would you like to do?': 'reset',
+    });
+    const state = createState(tmpDir);
+    await detectInstallStage(p, state, mockConfig);
+
+    expect(state.installAction).toBe('reset');
+    // Reset should NOT load existing values
+    expect(state.soul.agentName).toBeUndefined();
+  });
 });