Command runner & dry-run abstraction

[castrojo]

Description

Implement internal/runner — a wrapper around exec.Command that provides:

• Structured command execution with stdout/stderr capture
• --dry-run mode that logs commands without executing
• Fixture-driven testing support

Acceptance Criteria

• Runner interface with Run(ctx, cmd, args) (Output, error)
• DryRunner implementation that records commands without executing
• RealRunner implementation that executes and captures output
• Unit tests with fixture data
• All other packages (probe, install) use Runner — never raw exec.Command

Safety

This is the core safety abstraction. No real disk writes happen without going through Runner.

[castrojo]

Implementation Guidance (from architecture review)

Output Struct

type Output struct {
    Stdout   []byte
    Stderr   []byte
    ExitCode int
}

DryRunner as Test Spy

DryRunner should record all calls for assertion (test spy pattern):

type DryRunner struct {
    mu   sync.Mutex
    Calls []Call
}

type Call struct {
    Name string
    Args []string
}

func (d *DryRunner) Run(ctx context.Context, name string, args ...string) (Output, error) {
    d.mu.Lock()
    defer d.mu.Unlock()
    d.Calls = append(d.Calls, Call{Name: name, Args: args})
    return Output{}, nil
}

Example Call Site (how probe uses Runner)

// internal/probe/disks.go
func (p *Prober) Disks(ctx context.Context) ([]Disk, error) {
    out, err := p.runner.Run(ctx, "lsblk", "--json", "--output",
        "NAME,SIZE,TYPE,MODEL,SERIAL,TRAN,PATH,RM")
    if err != nil {
        return nil, fmt.Errorf("lsblk: %w", err)
    }
    return parseLsblkJSON(out.Stdout)
}

Error Wrapping Convention

Errors from Runner MUST include:

• The command name (lsblk, flatcar-install)
• The exit code
• stderr content (first 512 bytes max)

type CommandError struct {
    Name     string
    Args     []string
    ExitCode int
    Stderr   string
}

Missing from original issue — also implement:

• Environment variable passthrough (Run should accept optional env []string)
• Stdin handling (needed if flatcar-install prompts — pass io.Reader or nil)
• log/slog integration: log commands at Debug level, dry-run at Info level

[castrojo]

Implemented in commits 8f7422e..174b76f. All packages built, tested, and verified in QEMU VM.