feat(config): dotenv file loading

[CalvinAllen]

Description

Automatically load environment variables from .env files.

Proposed Syntax

# Per-task
build:
  dotenv: .env.production
  cmd: npm run build

# Global
config:
  dotenv: true    # Auto-load .env if present

Acceptance Criteria

• Load .env file automatically (optional)
• Support per-task dotenv file specification
• Parse standard dotenv format
• Don't fail if file doesn't exist (configurable)