Dead code in pkg/errors: Fatal()'s os.Exit() is unreachable; four ErrorX constants unused

Nobody has claimed this yet.

## Description

`pkg/errors/error.go` contains two related forms of dead code that mislead
readers about how the CLI signals failure.

### 1. `os.Exit(exitcode)` is unreachable

```go
func Fatal(exitcode int, args ...interface{}) {
log.Fatal(args...) // already calls os.Exit(1) internally
os.Exit(exitcode) // unreachable
}
```

Go's stdlib defines `log.Fatal` as:

```go
// log/log.go
func Fatal(v ...any) {
std.Output(2, fmt.Sprint(v...))
os.Exit(1)
}
```

Execution never reaches `os.Exit(exitcode)`. The `exitcode` argument passed by
callers is silently discarded — the binary always exits with code 1.

### 2. Four of the five `Error*` constants are unreferenced

Grep across the whole repo:

| Constant | Value | References outside `error.go` |
| --------------------------- | ----- | ----------------------------- |
| `ErrorCommandSpecific` | 1 | 0 |
| `ErrorConnectionFailure` | 11 | 0 |
| `ErrorAPIResponse` | 12 | 0 |
| `ErrorResourceDoesNotExist` | 13 | 0 |
| `ErrorGeneric` | 20 | 1 (CheckError, same file) |

The constants suggest a differentiated-exit-codes design that was never
actually wired up (the only caller of `Fatal` always passes `ErrorGeneric`,
and the chosen code is never reached anyway because of issue 1).

## Impact

- Misleading: reading the file gives the false impression that microcks-cli
returns differentiated exit codes (11/12/13/20). It always returns 1.
- Dead code rot: future readers may write callers expecting these codes to
matter.
- Lint noise: tools like `staticcheck` flag unreachable code (SA4006).

## Proposed fix

Two paths:

**(A) Implement differentiated exit codes properly.** Replace `log.Fatal` with
`log.Println`, let `os.Exit(exitcode)` actually run, audit every
`CheckError` call site to pass the right constant. This would alter
observable CLI exit codes (currently always 1), which could break downstream
CI scripts.

**(B) Remove the dead code, keep behavior identical.** Strip the unreachable
line, the unused constants, and the misleading `exitcode` parameter on
`Fatal`. No behavioral change.

I'd like to propose **(B)** as the minimal, safe cleanup. (A) can be tracked
separately as a feature if maintainers want differentiated exit codes.

Happy to send a PR with (B) once approach is approved.

## Environment

- Branch: `master` (development version `1.0.3`)
- File: `pkg/errors/error.go`

Start with pkg/errors/error.go, then grep the repository for CheckError and Fatal call sites to confirm the unused constants and current exit behavior. Done means the agreed minimal cleanup is applied without changing CLI behavior, with staticcheck or the project’s available checks confirming no unreachable-code or unused-constant issues remain.

Written by the indexing model from the issue text.