Add optional Yield developer helper

[bigboateng]

Summary

• Make the manual Yield workflow the default onboarding path: install the runtime, create the workflow and fixtures, run doctor --test, then register the adapters.
• Add the explicit yskill helper install command to the shared runtime used by the npm, Python, Rust, and Go packages.
• Keep yskill bootstrap and npm create @operatorstack/yield as compatibility aliases.
• Expand the existing skills/yield-workflow-builder helper with learn, create, convert, check, repair, upgrade, and register modes.

Safety and verification

• Teach the relevant Yield primitive and preview exact files and commands before a mutation.
• Require explicit approval before create, convert, repair, upgrade, or register work.
• Keep learn and check read-only; static checks do not execute fixtures.
• Limit repair to two attempts and require doctor --test, registration, and adapter verification before successful mutations complete.
• Refuse path escapes, existing destinations, user-owned helper or adapter paths, helper self-upgrades during active runs, and unsupported Yield version changes.
• Verify that package installation alone does not materialize the helper or coding-agent adapters.

Compatibility and rollout

• Preserve the canonical skills/yield-workflow-builder name and existing generated ownership metadata.
• Preserve explicit bootstrap users without adding an install hook or migration.
• Reorder the README, SDK guides, quickstart, agent setup, conversion guide, and CLI reference so the optional helper appears after normal usage.
• Include a minor Changeset. The command should be advertised on the live documentation only after the matching package release is available.
• Users must install the helper explicitly and restart their coding agent after adapter registration.

Verification

• go test ./...
• npm run test:release — 46 tests passed
• npm run format:check
• Cross-language fixtures for all seven modes in TypeScript, Python, Go, and Rust
• Refusal coverage for cancellation, declined mutations, path escapes, existing destinations, ownership conflicts, self-upgrade, and unsupported versions