Add skip_assistant_name option to the send service

[kauelima]

Description

Adds an optional skip_assistant_name boolean to the universal_notifier.send service. When true, the assistant name is omitted from the visual prefix:

Flags	Result
none (default)	[Assistant - 10:00] Greeting. Message (unchanged)
skip_assistant_name: true	[10:00] Greeting. Message
+ include_time: false + skip_greeting: true	Message

When the prefix is empty, no brackets or leading space are emitted — this also fixes the [ - 10:00] artifact when the configured assistant name is empty. Voice/TTS and command_* messages are unaffected.

Changes: new constant in const.py, schema + handler + prefix logic in __init__.py, new field in services.yaml, 3 new tests, service field tables in README.md/README_IT.md, and a CHANGELOG.md entry.

Motivation and Context

There is no way to send a notification without the [Assistant Name] prefix — the name can be overridden but never removed, so mobile notifications always arrive as e.g. [Home Assistant] Good Morning. Your house is flooding. This adds the missing flag, matching the existing skip_greeting / include_time options.

How has this been tested?

• 3 new tests in tests/test_init.py covering: name removed with time kept, fully bare message (all flags), and name removed from the title prefix
• Full suite via the project's Docker test-runner: 126 passed, existing prefix/greeting/title tests unchanged
• Manual tests overriding the files in a Home Assistant instance to test the UI configuration and notification sent (tested on an Android companion app)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly. (did my best in Italian readme)