[codex] add VS Code worktree handoff

[dango85]

Summary

Adds Level 1 VS Code handoff guidance for the worktree-first specify flow.

Changes

• Adds VS Code handoff flags to /speckit.worktrees.specify: --open-vscode, --no-open-vscode, --vscode-new-window, --vscode-reuse-window, and --vscode-print-command.
• Documents config keys in worktree-config.yml: vscode_open_after_create, vscode_open_mode, and vscode_command.
• Adds README guidance for opening the created worktree via code -n .
• Records the change in the Unreleased changelog.

Rationale

This keeps the behavior extension-level and practical: the command can create/reuse the worktree and hand it to VS Code through the code CLI. It avoids claiming direct VS Code API control, and keeps new-window as the safe default while allowing explicit reuse-window opt-in.

Validation

• git diff --check
• ruby -e 'require "yaml"; YAML.load_file("worktree-config.yml"); YAML.load_file("extension.yml")'
• bash tests/test-post-install.sh
• bash tests/test-create-worktree.sh

Refs #4