Clarify queue package positioning

[samdark]

Summary

• Replace extension wording with framework-agnostic queue library wording in README.
• Add the same positioning to the guide index.
• Update the Composer package description accordingly.

Tests

• make composer NO_TTY=1 ARGS="validate --strict"

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (f6ebc0a) to head (e712ec8).

Additional details and impacted files

@@           Coverage Diff            @@
##             master    #295   +/-   ##
========================================
  Coverage      0.00%   0.00%           
  Complexity      330     330           
========================================
  Files            49      49           
  Lines           905     905           
========================================
  Misses          905     905           

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[Copilot]

Pull request overview

Updates the project’s public-facing messaging to position yiisoft/queue as a framework-agnostic PHP queue library (while noting it fits best with Yii 3), aligning README, docs index, and Composer metadata.

Changes:

• Replaces “extension” phrasing with framework-agnostic queue library wording in the root README.
• Adds the same positioning statement to the documentation index page.
• Updates composer.json package description to match the new positioning.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
README.md	Updates the one-line package positioning/description.
docs/guide/en/README.md	Adds the same positioning statement to the guide index.
composer.json	Updates Packagist/Composer package description to reflect new wording.

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