fix: stop command missing --name flag to specify which instance to stop

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Why we need this:
The start command supports --name flag to name a Microcks instance
but the stop command has no --name flag. Users cannot specify which
instance to stop when multiple instances exist.

How this will help:
Adding --name flag to stop command makes it consistent with start
command and allows users to stop a specific named instance.

## Problem
microcks start --name myinstance ✅ works
microcks stop --name myinstance ❌ flag does not exist

The stop command always resolves the current context instead of
allowing the user to specify which instance to stop by name.

## Fix
- Add --name flag to stop command
- Pass name to ResolveContext() so correct instance is stopped
- Remove unnecessary empty fmt.Println() call

Start at the stop command entry point and compare its flags and context resolution with the start command, focusing on ResolveContext(). Verify the command help exposes --name, confirm the name reaches context resolution, and check that stopping a named instance works without the unnecessary empty output.

Written by the indexing model from the issue text.