Issue Description

Add a maturity field to the extension collection manifest schema so that entire collections can be marked as experimental or deprecated, controlling whether they are released in PreRelease and/or Stable channels.

Motivation

Individual artifacts already carry maturity levels (stable, preview, experimental, deprecated) in the AI Artifacts Registry. However, no equivalent concept exists at the collection level. When a new persona collection is being developed or tested, it should be possible to mark the entire collection as experimental so it is only packaged and published as a PreRelease extension, never as a Stable release. Likewise, when a collection is retired, marking it deprecated should prevent it from being released in either channel.

This enables a safe lifecycle for persona collections:

1. experimental: Collection is under active development. Built and published as PreRelease only. Users who install the PreRelease channel receive it; Stable channel users never see it.
2. preview: Collection is feature-complete but not yet validated at scale. Built for both PreRelease and Stable channels (same as artifact-level preview behavior).
3. stable: Collection is production-ready. Built and published in both channels (default, current behavior).
4. deprecated: Collection is retired. No longer built or published in any channel. Existing installations remain but receive no updates.

Deliverables

1. Collection Manifest Schema Update

Update scripts/linting/schemas/collection.schema.json to add an optional maturity property:

"maturity": {
    "type": "string",
    "enum": ["stable", "preview", "experimental", "deprecated"],
    "default": "stable",
    "description": "Collection-level maturity controlling release channel eligibility. Experimental collections are released only as PreRelease. Deprecated collections are excluded from all releases."
}

The field is optional with a default of stable to maintain backward compatibility with existing collection manifests that omit it.

2. Collection Manifest Updates

Add maturity field to existing and future collection manifests in extension/collections/:

3. Packaging Script Integration

Update scripts/extension/Package-Extension.ps1 and scripts/extension/Prepare-Extension.ps1 to:

• Read the maturity field from the collection manifest
• When building for a Stable release channel, skip collections with experimental maturity
• When building for any channel, skip collections with deprecated maturity
• Log a clear message when a collection is skipped due to maturity gating

Channel-to-maturity eligibility matrix:

4. CI/CD Workflow Integration

Update the extension packaging workflow (.github/workflows/extension-package.yml) to:

• Pass the release channel context to packaging scripts
• Respect collection maturity when determining which collections to build in the matrix strategy
• Ensure deprecated collections are excluded from the build matrix entirely

5. Validation Integration

Update scripts/linting/Validate-ArtifactRegistry.ps1 or add collection-level validation to:

• Validate that collection manifest maturity values are valid enum members
• Warn when a deprecated collection still exists in the build matrix
• Warn when an experimental collection has been experimental for longer than a configurable threshold (optional, future)

Acceptance Criteria

• Collection manifest schema supports an optional maturity field with enum values stable, preview, experimental, deprecated
• Existing collection manifests without maturity default to stable (backward compatible)
• Packaging scripts skip experimental collections when building for Stable channel
• Packaging scripts skip deprecated collections for all channels
• CI/CD build matrix respects collection maturity gating
• Validation catches invalid maturity values in collection manifests
• A collection can transition from experimental to stable by changing a single field

Technical Notes

• The collection-level maturity is independent of artifact-level maturity. A stable collection can contain preview artifacts (which are filtered by the existing artifact-level channel logic). The collection maturity gates the entire package, while artifact maturity gates individual files within it.
• The deprecated state is terminal: once set, the collection should not be republished. Consider adding a workflow guard that fails the build if a deprecated collection is accidentally included.
• This aligns with the existing maturity enum values used in the AI Artifacts Registry schema (stable, preview, experimental, deprecated), maintaining vocabulary consistency across the system.

Additional Context

• Collection schema: scripts/linting/schemas/collection.schema.json
• Existing collection manifests: extension/collections/
• Registry maturity enum: scripts/linting/schemas/ai-artifacts-registry.schema.json (artifact-level precedent)
• Packaging scripts: scripts/extension/Prepare-Extension.ps1, scripts/extension/Package-Extension.ps1
• CI workflow: .github/workflows/extension-package.yml
• Depends on: Phase 2 (feat: Multi-Package Extension Build System #433, Multi-Package Build System) for collection-aware packaging
• Depends on: Phase 5 (feat: Publishing Infrastructure and CI/CD for Multi-Package Distribution #436, Publishing Infrastructure) for CI/CD multi-package workflow
• Parent epic: feat: Persona-Based Extension Distribution #431 (Persona-Based Extension Distribution)