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>

.claude-plugin/marketplace.json

@@ -11,7 +11,7 @@
       "name": "ab",
       "source": "./plugin",
       "description": "Spec-driven build orchestration with git worktree isolation and session memory",
-      "version": "1.0.3",
+      "version": "1.0.4",
       "keywords": ["build", "spec", "automation", "worktree", "qa"]
     }
   ]