Publish a deprecation and compatibility lifecycle policy

[Scriptception]

Problem

Project and skill versions exist, but users do not have one contract for agent support changes, skill deprecation, metadata evolution, breaking CLI changes, or security fixes.

User outcome

Users and maintainers can upgrade confidently and understand how long old formats and skills remain supported.

Scope

• Define semantic versioning responsibilities for the package and individual skills.
• Define deprecation notice, replacement, removal, and urgent security-fix paths.
• Define compatibility expectations for catalog, stamps, adapters, and lockfiles.
• Add lifecycle fields to metadata where needed.

Non-goals

• Promising indefinite support for every agent.
• Changing existing skill content.

Acceptance criteria

• The policy covers package, skill, adapter, metadata, and install-state changes.
• Deprecated skills remain discoverable with a replacement or rationale.
• Breaking changes include migration guidance and a minimum notice rule except urgent security cases.
• Release checks require lifecycle notes when compatibility changes.

Validation

• Walk through one simulated skill rename, one agent removal, and one stamp-format migration.
• Verify the public catalog can represent each lifecycle state.

Relationships

• Uses the compatibility matrix and machine-readable catalog.
• Required before team lockfiles are stable.