docs(contributing): align tool discovery and test runner with AGENTS.md

[xxxigm]

What does this PR do?

Updates CONTRIBUTING.md so contributor guidance matches how the repo actually works:

• Adding a new tool: Removes the outdated instruction to add imports to a _modules list in model_tools.py (that list does not exist). Documents that built-in tools in tools/*.py are auto-discovered via discover_builtin_tools() in tools/registry.py, and that wiring the tool name into toolsets.py is still required. Points readers to AGENTS.md (“Adding New Tools”) for the full workflow.
• Running tests: Recommends scripts/run_tests.sh (CI-parity) as the preferred runner, with pytest tests/ -v as an alternative, and aligns the “Before submitting” checklist with the same guidance.

Related Issue

Fixes #

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

1. CONTRIBUTING.md — “Run tests”: document scripts/run_tests.sh as preferred vs pytest.
2. CONTRIBUTING.md — “Adding a New Tool”: replace _modules / model_tools.py import list with auto-discovery + required toolsets.py wiring + link to AGENTS.md.
3. CONTRIBUTING.md — “Before submitting”: recommend scripts/run_tests.sh for CI parity.

How to Test

1. Open CONTRIBUTING.md on this branch and confirm the three sections read correctly and match AGENTS.md / tools/registry.py behavior.
2. (Optional) Run scripts/run_tests.sh — no behavior change expected; doc-only PR.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform:

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

For New Skills

• This skill is broadly useful to most users (if bundled) — see Contributing Guide
• SKILL.md follows the standard format (frontmatter, trigger conditions, steps, pitfalls)
• No external dependencies that aren't already available (prefer stdlib, curl, existing Hermes tools)
• I've tested the skill end-to-end: hermes --toolsets skills -q "Use the X skill to do Y"

Screenshots / Logs