Add context-aware shell helper package (fork of mage/sh)

[Copilot]

Summary

This PR creates a context-aware fork of github.com/magefile/mage/sh as a standalone Go package at github.com/magefile/shx.

What changed

Every exported function that runs a command now accepts a context.Context as its first parameter. The context is passed through to exec.CommandContext, giving callers proper control over timeouts, cancellation, and deadlines.

Key design decisions

• context.Context on all command functions — Run, RunV, RunWith, RunWithV, Output, OutputWith, Exec all take ctx as their first parameter. The closures returned by RunCmd and OutCmd also accept a context.
• No mage/mg dependency — The mg.Verbose() check is inlined (reads MAGEFILE_VERBOSE env var directly) and mg.Fatalf() is replaced with a local fatalError type that implements the same ExitStatus() int interface. This makes shx a zero-dependency package.
• File helpers unchanged — Rm and Copy don't execute commands, so they don't need a context parameter.
• New test — TestContextCancellation verifies that a cancelled context properly terminates a running command.

Files

File	Description
cmd.go	Core command-running functions with context.Context support
helpers.go	File helpers (Rm, Copy) — unchanged from original
cmd_test.go	Tests for command functions, adapted for context
helpers_test.go	Tests for file helpers
testmain_test.go	Test binary helper infrastructure
go.mod	Go module definition
README.md	Package documentation with usage examples

Testing

All 17 tests pass:

ok  	github.com/magefile/shx	0.011s