feat(registry): asset lifecycle (publish, deprecate, retire, migrate)

**Context:** ROADMAP.md → Asset lifecycle management

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## Component

API or orchestration

## Describe the feature

**Publish, deprecate, retire** flow for registry assets. Deprecation warnings in task telemetry for pinned versions nearing EOL. Retirement blocks new tasks referencing the asset. Operator API for **bulk migration** (re-pin all blueprints v1 → v2).

## Use case

Operators must sunset vulnerable or obsolete MCP servers/skills without breaking running tasks or manually editing every Blueprint.

## Proposed solution

1. State machine: `published → deprecated → retired`.
2. Task telemetry `asset_deprecated_warning` when pinned version deprecated.
3. `createTask` rejects retired assets with clear error.
4. `POST /registry/migrate` bulk re-pin operator action.
5. CLI: `bgagent registry deprecate|retire|migrate`.

## Other information

- Child of #246.
- Depends on asset versioning draft.

- [ ] This might be a breaking change

Start with the Asset lifecycle management entry in ROADMAP.md and review the asset versioning draft it depends on. Trace the createTask path, task telemetry, the registry API, and the bgagent registry CLI commands named in the proposal. Done means the publish/deprecate/retire flow, deprecation warning, retired-asset rejection, bulk migration endpoint, and CLI actions are defined and covered.

Written by the indexing model from the issue text.