fix: syncDirectory — guard same-path copy and skip nested .git dirs

Two bugs causing 'EACCES: permission denied, copyfile' when source
and target are the same path (e.g. wizard with sourceDir == mosaicHome):

1. No same-path guard — syncDirectory tried to copy every file onto
   itself; git pack files are read-only (0444) so copyFileSync fails.
2. excludeGit only matched top-level .git — nested .git dirs like
   sources/agent-skills/.git were copied, hitting the same permission
   issue.

Fixes:
- Early return when resolve(source) === resolve(target)
- Match .git dirs at any depth via dirName and relPath checks
- Skip files inside .git/ paths

Added file-ops.test.ts with 4 tests covering all cases.

packages/mosaic/src/platform/file-ops.ts

@@ -9,7 +9,7 @@ import {
   unlinkSync,
   statSync,
 } from 'node:fs';
-import { dirname, join, relative } from 'node:path';
+import { dirname, join, relative, resolve } from 'node:path';
 
 const MAX_BACKUPS = 3;
 
@@ -68,6 +68,9 @@ export function syncDirectory(
   target: string,
   options: { preserve?: string[]; excludeGit?: boolean } = {},
 ): void {
+  // Guard: source and target are the same directory — nothing to sync
+  if (resolve(source) === resolve(target)) return;
+
   const preserveSet = new Set(options.preserve ?? []);
 
   // Collect files from source
@@ -77,9 +80,10 @@ export function syncDirectory(
     const stat = statSync(src);
     if (stat.isDirectory()) {
       const relPath = relative(relBase, src);
+      const dirName = relPath.split('/').pop() ?? '';
 
-      // Skip .git
-      if (options.excludeGit && relPath === '.git') return;
+      // Skip any .git directory (top-level or nested, e.g. sources/agent-skills/.git)
+      if (options.excludeGit && (dirName === '.git' || relPath.includes('/.git'))) return;
 
       // Skip preserved paths at top level
       if (preserveSet.has(relPath) && existsSync(dest)) return;
@@ -91,6 +95,9 @@ export function syncDirectory(
     } else {
       const relPath = relative(relBase, src);
 
+      // Skip files inside .git directories
+      if (options.excludeGit && relPath.includes('/.git/')) return;
+
       // Skip preserved files at top level
       if (preserveSet.has(relPath) && existsSync(dest)) return;