feat: Clear next steps after worktree creation

When user chooses to create a worktree, build.md now displays a clear
"ACTION REQUIRED" message explaining:
- What was created (worktree path, branch, plan)
- Exact command to open new VSCode window in worktree
- How to continue the build in the new window
- Why switching is necessary (Claude Code is directory-bound)

Bumps version to 1.0.4.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugin/.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "ab",
   "description": "Auto-Build: Spec-driven build orchestration with worktree isolation and session memory",
-  "version": "1.0.3",
+  "version": "1.0.4",
   "author": {
     "name": "ROA2WEB Team"
   }