feat: add Agent QA tester subagent

[pranshuchittora]

Summary

Adds agent-qa-tester, a Codex-native quality subagent for operating Agent QA, the self-improving QA agent for software teams.

The subagent covers three tightly related Agent QA workflows:

• author schema-valid web/mobile tests, suites, and hooks with generated canonical IDs;
• triage failed runs from steps, artifacts, and logs rather than guessed UI state;
• apply the smallest authorized code or YAML fix and verify the original failure path.

It prefers an already-configured Agent QA MCP server, falls back to an existing local CLI, and explicitly avoids installing software, mutating production-facing targets, inventing evidence, or hiding product defects without parent-agent authorization.

This is distinct from the general qa-expert and test-automator: it is an operator for Agent QA's concrete MCP/CLI contracts, run evidence, validation flow, and self-healing boundary.

Validation

• Parsed all 173 category TOML files successfully with Python tomllib.
• Verified the new name is unique and the main/category links resolve locally.
• git diff --check passes.

Pull Request Checklist

• Added categories/04-quality-security/agent-qa-tester.toml
• Updated main README.md
• Updated category README.md
• Verified links and TOML validity
• Included clear PR description and motivation