Refactor: Improve clarity and readability of run-atpull log message

[alberti42]

Description:

This pull request addresses two issues with the log message displayed when an update proceeds without new commits due to the run-atpull ice or the --urge option.

1. The Problem:

• Confusing Wording: The original message, "Continuing with the update because 'run-atpull'' ice given," was grammatically awkward and didn't clearly explain what was happening (i.e., that no new commits were found).
• Poor Readability: The message used the {msg3} color code, a dark gray (#38;5;238m), which is unreadable on dark terminal backgrounds.

2. The Solution:

This PR refactors the message in zinit-install.zsh to be clearer, more informative, and visually consistent with other Zinit output.

• The message now explicitly states that no new commits were found but that post-update hooks are being run as requested.
• The unreadable {msg3} color has been replaced with {rst}, which resets the color to the terminal's default foreground, guaranteeing visibility on any background.
• The now-unused col-msg3 definition has been removed from zinit.zsh for cleanup.

Before:

(On a dark background, the text after "because" is nearly invisible)

[update] Continuing with the update because run-atpull'' ice given.

After:

(Clear, readable, and consistent with other Zinit messages)

[update] No new commits found, but running post-update hooks as requested by the 'run-atpull' ice.

or, when using the --urge flag:

[update] No new commits found, but running post-update hooks as requested by the -u/--urge option.

[pschmitt]

Thanks!