Migrate otdfctl into platform monorepo

[alkalescent]

Summary

We are planning to migrate the otdfctl CLI from this standalone repository into the opentdf/platform monorepo. After migration, this repository will be archived and marked read-only.

Why

• otdfctl already depends heavily on platform (SDK, protocol, libs) and uses platform's reusable CI workflows
• Both repos run each other's e2e tests in CI — consolidating eliminates cross-repo coordination overhead
• The platform monorepo already supports per-component releases (service, sdk, libs), so otdfctl can maintain independent release cadence

What changes for users

• Go module path will change from github.com/opentdf/otdfctl to github.com/opentdf/platform/otdfctl
• Release tags will change from v0.X.Y to otdfctl/v0.X.Y
• This repository will be archived (read-only) — all existing releases and tags will remain accessible
• A notice will be added to this README pointing to the new location

What stays the same

• The otdfctl binary name and CLI interface
• Separate release cadence (not coupled to platform service releases)
• All existing CI tests continue to run

Feedback

If you have concerns or questions about this migration, please comment on this issue.

[marythought]

hi @alkalescent , can you please post in https://github.com/orgs/opentdf/discussions where more people are likely to see? Feel free to link to the issue so as not to repeat yourself! But solicitations for comments should happen there.