docs(legal): prepare contributor agreement draft

[Codewriter90x]

Summary

Prepares a non-operative Harmony-based Contributor License Agreement planning draft and a separate adoption checklist for HitTheKit's possible future dual-licensing model.

The proposed planning model is a copyright license, not an assignment: contributors retain copyright while the draft evaluates the copyright and patent permissions needed for community and separate commercial/proprietary distribution.

Important status

• NOT legally reviewed or approved.
• NOT in force and must not be accepted, signed, or relied upon.
• No CLA Assistant or electronic acceptance process is configured.
• Substantial external code and asset contributions remain paused.
• Opening a pull request, issue, or discussion does not accept the draft.
• This PR does not change the repository license or contribution policy.

Human review required

The checklist intentionally leaves adoption blockers unresolved, including the receiving legal party, address/contact, governing law and forum, privacy/data retention, electronic signatures, employer/entity and minor contributors, patent scope, prior contributions, and the exact GPL-to-MPL transition.

Validation

• public-readiness contract tests: passed
• asset-provenance contract tests: passed
• relative Markdown link validation: passed
• git diff --check: passed
• sensitive-pattern scan: passed
• full .NET suite: not run with the pinned SDK because SDK 8.0.411 is unavailable in this checkout environment; this documentation-only change is covered by repository CI

Scope

Documentation only. No merge, no CLA activation, no CLA Assistant configuration, and no relaxation of the current contribution block.