feat: add repository-testing agent for post-install verification

New agent that users can run to verify their IDAD installation:

Tests performed:
1. File structure - checks all agent files and rules exist
2. Workflow configuration - verifies correct CLI paths and secrets
3. GitHub labels - ensures required labels are created
4. Repository secrets - confirms API keys are configured
5. Actions permissions - checks workflow permissions

Optional tests:
6. Workflow trigger - actually triggers a workflow run
7. End-to-end test - creates a test issue with idad:auto

Usage:
  # Cursor
  cursor-agent -f .cursor/rules/system.mdc -f .cursor/agents/repository-testing.md -p "Run tests"

  # Claude
  claude --system-prompt "$(cat .claude/rules/system.md)" -p "Run tests..."

Author: kidrecursive