feat(cli): interactive task selection

[CalvinAllen]

Description

Show an interactive task picker when run without arguments.

Behavior

./rnr
# Shows interactive list:
# > build          Build the project
#   test           Run tests
#   deploy         Deploy to production
#   lint           Run linter
# Use arrow keys to select, Enter to run

Acceptance Criteria

• Show when no task specified (and no default task)
• List all available tasks with descriptions
• Arrow key navigation
• Enter to select and run
• Escape/Ctrl+C to cancel
• Fuzzy search/filter

Notes

Consider using dialoguer or inquire crate for terminal UI.