feat(cli): default task

[CalvinAllen]

Description

Run a default task when no task name is specified.

Proposed Syntax

default: build

build:
  cmd: cargo build

Usage

./rnr           # Runs 'build'
./rnr build     # Also runs 'build'

Acceptance Criteria

• Support default key at root level
• Run specified task when ./rnr is invoked without arguments
• Show helpful message if no default and no task specified