Add contributing entrypoint

[jaysinailabs]

Summary

• add a GitHub-recognized CONTRIBUTING.md entrypoint that routes contributors to the canonical CogLang contribution guide, roadmap, PR template, and external host/consumer issue template
• include the entrypoint in the public repo extract manifest, required manifest destinations, package data, and _public_assets mirror
• add tests that the entrypoint is materialized and preserves the external host/consumer boundary route

Validation

• ..venv\Scripts\python.exe -m pytest tests\coglang\test_open_source_extract.py tests\coglang\test_public_assets_mirror.py tests\coglang\test_cli.py::test_cli_public_repo_extract_manifest_payload_shape -q
• ..venv\Scripts\python.exe -m coglang.cli public-assets --format text
• ..venv\Scripts\python.exe -m coglang.cli release-check --format text
• git diff --check
• ..venv\Scripts\python.exe -m pytest -q

Versioning

No package version bump. This is part of the source-HEAD maintenance batch after PyPI 1.1.4.