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"
   }

plugin/commands/build.md

@@ -37,7 +37,37 @@ Create an isolated git worktree? (recommended for larger features)
 If yes:
 - Run: `bash ${CLAUDE_PLUGIN_ROOT}/scripts/worktree-create.sh {feature-name}`
 - Update status.json with worktree path
-- Inform user of worktree location
+- **IMPORTANT**: After worktree creation and planning, display clear next steps:
+
+```
+════════════════════════════════════════════════════════════════
+  WORKTREE CREATED - ACTION REQUIRED
+════════════════════════════════════════════════════════════════
+
+✅ Worktree created at: ../ab-worktrees/{project}-{feature-name}/
+✅ Branch: feature/ab-{feature-name}
+✅ Plan created: .auto-build-data/specs/{feature-name}/plan.md
+
+⚠️  NEXT STEPS (you must do this to continue):
+
+1. Open a NEW VSCode window in the worktree:
+
+   code ../ab-worktrees/{project}-{feature-name}/
+
+2. In the NEW window, continue the build:
+
+   /ab:build {feature-name}
+
+   (It will detect the plan and start implementation)
+
+────────────────────────────────────────────────────────────────
+Why? Claude Code sessions are tied to the current directory.
+To work in the isolated worktree, you need a new session there.
+════════════════════════════════════════════════════════════════
+```
+
+- **STOP HERE** - Do not proceed with implementation in the current directory
+- The user must switch to the worktree directory first
 
 ### 3. Planning Phase