Add support for Gatling -> 152-java-performance-gatling

[jabrena]

User Story: 152-java-performance-gatling

As a Java developer using this cursor-rules-java project
I want to add a new 152-java-performance-gatling skill for Gatling-based performance testing
So that I can generate consistent guidance and project scaffolding for running Gatling load tests in Java projects, alongside the existing JMeter performance-testing skill.

Acceptance Criteria

Feature: Gatling performance testing skill

Background:
  Given the repository generates skills from XML sources under "skills-generator/src/main/resources"
  And generated content under "skills/" is not edited directly

@acceptance-test
Scenario: Generate the Gatling performance testing skill from XML sources
  Given a new Gatling skill is defined in the skill generator sources
  When the skills-generator module is installed
  Then the generated skills include "152-java-performance-gatling"
  And the skill explains when to use Gatling performance testing
  And the skill provides practical steps for adding and running Gatling simulations in a Java project

@integration-test
Scenario: Register the Gatling skill in the skill catalog
  Given the Gatling skill XML source exists
  When the skill index and skill list are generated
  Then "152-java-performance-gatling" appears in the generated skills inventory
  And the skill can be discovered with a prompt such as "Add Gatling performance testing to this project"

@integration-test
Scenario: Keep generated artifacts reproducible
  Given the Gatling skill is generated from XML
  When "./mvnw clean verify -pl skills-generator" is executed
  Then the build succeeds
  And no generated skill content requires manual edits under "skills/"

@integration-test
Scenario: Validate generated skill quality
  Given the generated Gatling skill exists under "skills/"
  When "npx skill-check@latest skills --no-security-scan --format github" is executed
  Then the skill passes the formatter and structural checks
  And any behavioral scan findings are reviewed before promoting the change

Notes

• Use the existing 151-java-performance-jmeter skill as the nearest reference for scope, naming, and generated-skill structure.
• Implement changes in XML sources under skills-generator/src/main/resources/, including the relevant skill reference, skill index, and catalog wiring.
• Do not edit generated skills/ content directly; regenerate it through the Maven pipeline.
• The skill should focus on Gatling setup and execution for Java projects, including where simulations live, how to run them, and what outputs users should expect.
• Keep the implementation compatible with the repository guidance for Java 25 and Maven-based workflows.

INVEST Validation

• Independent: The Gatling skill can be added as a new performance-testing skill without changing the existing JMeter skill behavior.
• Negotiable: Exact script names, Gatling execution approach, and output paths can be refined while preserving the intent of Gatling performance-test support.
• Valuable: Java developers gain a ready-to-use skill for Gatling load testing, expanding the performance-testing options in the project.
• Estimable: The work is bounded by creating and wiring one generated skill plus validation through the existing generator and skill-check workflows.
• Small: The scope should fit in one iteration because it mirrors an existing neighboring skill pattern.
• Testable: Completion can be verified by generated skill output, catalog discovery, Maven verification, and skill validation commands.

[jabrena]

#763