feat(runner): variable interpolation

[CalvinAllen]

Description

Support variables in task definitions.

Proposed Syntax

vars:
  version: "1.0.0"
  name: myapp

build:
  cmd: cargo build --version \${{ vars.version }}

tag:
  cmd: git tag \${{ vars.name }}-v\${{ vars.version }}

Acceptance Criteria

• Define variables in vars section
• Interpolate in cmd strings
• Support environment variable references
• Support nested variable references