Feature Request

Problem

After creating a worktree, gtr displays helpful "Next steps" like:

Next steps:
  cd /path/to/worktree
  git gtr editor feature-branch

However, users cannot add project-specific next steps (e.g., npm install, cp .env.example .env).

While postCreate hooks exist, their output appears before the "Next steps" section, making it less visible and disconnected from the workflow guidance.

Proposed Solution

Add gtr.nextStep multi-valued configuration option to display custom next steps alongside the built-in ones:

git gtr config add gtr.nextStep "npm install  # Install dependencies"
git gtr config add gtr.nextStep "cp .env.example .env  # Setup environment"

Output after creating worktree:

Next steps:
  cd /path/to/worktree
  git gtr editor feature-branch
  npm install  # Install dependencies
  cp .env.example .env  # Setup environment

Benefits

• Custom steps appear in the logical place (with other next steps)
• Supports variable substitution ($BRANCH, $WORKTREE_PATH)
• Can be shared via .gtrconfig for team consistency