Add repository-specific hooks support

[gage-halverson-fetch]

Summary

Adds a hooks system that allows repositories to define custom scripts executed during sprout operations.

• HooksService discovers and executes hooks from .sprout/hooks/ within each repository
• post-prune hook runs after each worktree is removed, receiving environment variables:
  • SPROUT_WORKTREE_PATH - path of the removed worktree
  • SPROUT_BRANCH - branch name that was pruned
  • SPROUT_REPO_ROOT - root directory of the main repository
• Dry-run mode shows which hooks would run
• Verbose flag (-v) for detailed hook execution output

Example Use Case

Clean up Xcode DerivedData when pruning worktrees from iOS projects:

# .sprout/hooks/post-prune
#!/bin/bash
for dir in ~/Library/Developer/Xcode/DerivedData/Fetch-*/; do
  workspace=$(plutil -extract WorkspacePath raw "$dir/info.plist" 2>/dev/null)
  if [[ "$workspace" == *"$SPROUT_WORKTREE_PATH"* ]]; then
    rm -rf "$dir"
    echo "Cleaned: $(basename $dir)"
  fi
done

Test plan

• Build passes
• Dry-run shows "Would run post-prune hook" when hook exists
• Hook executes after worktree removal
• Non-executable hooks show warning with chmod instructions

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