Add a testable Run(...) int CLI entry point

[MPV]

What

Replace cmd.Execute(args) with a single testable entry point:

func Run(args []string, stdin io.Reader, stdout, stderr io.Writer) int
func main() { os.Exit(cmd.Run(os.Args[1:], os.Stdin, os.Stdout, os.Stderr)) }

Run resolves the arguments, writes images to the injected stdout, reports errors to the injected stderr (via a local log.New(stderr, "", 0) rather than the global logger), and returns the exit code. processor.ProcessStdin now takes an io.Reader so stdin is injectable too.

Why

The CLI's observable contract — argv + stdin → stdout, stderr, exit code — could not be asserted in-process: Execute wrote to os.Stdout, read os.Stdin, and exited via log.Fatal. Tests had to hijack os.Stdout/os.Args. Making the exit code a returned value and the streams injected parameters means the whole triple is now assertable from a plain function call — the enabler for keeping the e2e/triple goldens in approvals/ in-process (no subprocess, no go build).

Behavior change

None — verified byte-for-byte. Built the binary and compared against master across every scenario: single file, multi-document file, stdin, unsupported kind (stderr + exit 0), missing path (silent + exit 0), and no-args usage (stderr + exit 1). Identical stdout/stderr/exit in all cases.

The cmd tests are rewritten to call Run with bytes.Buffers and assert the triple directly — no os.Stdout/os.Stdin/os.Args swapping, no chdir.

Relationship to the open CLI stack

This supersedes #50 (Execute(args, out io.Writer) error): Run is the fuller version — it returns an exit code and injects stdin/stderr as well. The behavior changes in the other open PRs are orthogonal and would layer cleanly on top of Run rather than on Execute:

• Exit non-zero when a file cannot be processed #55 (exit non-zero on file failure) → have the per-file loop count failures and return 1.
• Error on file arguments that match nothing #57 (error on missing path) → unchanged (fileutil).
• Process all documents from stdin, not just the first #49 (stdin multi-doc) / Skip non-workload documents instead of aborting the stream #54 (skip non-workloads) → unchanged (processor/yamlparser).

If you adopt Run, the cleanest path is to close/rework #50 and re-base #55 onto this shape — say the word and I'll reorganize that stack. No dependency change; -race/vet/gofmt/go mod tidy all clean.

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https://claude.ai/code/session_01Pc6NAURAqjU4LYJx93tgSC

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