refactor(gem-browser-tester): restructure agent with detailed workflo…

[mubaidr]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• My contribution adds a new instruction, prompt, agent, skill, or workflow file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, agent, skill, or workflow with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.

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Description

Overhauled the browser tester agent specification with a comprehensive, step-by-step workflow and explicit input/output contracts. Key changes:

• Removed mission statement and refined expertise scope
• Replaced high-level workflow with detailed procedural steps:
  • Explicit initialization, execution, verification, failure handling, reflection, and cleanup phases
  • Mandated accessibility snapshots over visual screenshots for reliable element identification
  • Defined structured evidence collection in docs/plan/{plan_id}/evidence/{task_id}/
• Added JSON input format guide defining task_id, plan_id, plan_path, and task_definition
• Added JSON output format guide with standardized status, summary, and failure details

This refactor makes agent behavior predictable, ensures consistent evidence collection, and enables reliable integration with downstream systems through well-defined I/O schemas.

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Type of Contribution

• New instruction file.
• New prompt file.
• New agent file.
• New plugin.
• New skill file.
• New agentic workflow.
• Update to existing instruction, prompt, agent, plugin, skill, or workflow.
• Other (please specify):

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Additional Notes

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By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.