feat(registry): capability descriptors for dynamic environment config

**Context:** ROADMAP.md → Capability descriptors

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## Component

Agent (Python runtime)

## Describe the feature

Structured **metadata per registry asset** declaring:

**Provides:** tool surface, permissions required, resource limits, Cedar actions introduced.

**Requires:** runtime dependencies, network egress domains, minimum compute profile.

Agent runtime uses descriptors to configure the execution environment dynamically—MCP servers, context injection, sandbox permissions—rather than static blueprint lists. Registry is evaluated by Change Manifest **L1** (`tool_needed` validation) and **L5** (knowledge resolution).

## Use case

Static blueprint lists do not scale as asset types grow. Runtime needs machine-readable requirements to provision MCP servers, context injection, and sandbox permissions.

## Proposed solution

1. JSON Schema for `CapabilityDescriptor` stored alongside asset artifact.
2. Resolver validates descriptor at task start; fails closed on mismatch with compute profile.
3. Gateway + Cedar auto-generate allowlists from descriptor actions.
4. Validator in registry publish path.
5. Golden fixtures in `contracts/` or `cdk/test/`.

## Other information

- Child of #246; pairs with extended tool capability tiers draft.
- Design context: `docs/design/COMPUTE.md`.

- [ ] This might be a breaking change

Empieza por ROADMAP.md y docs/design/COMPUTE.md; después inspecciona la ruta de publicación del registry y las ubicaciones contracts/ o cdk/test/ mencionadas para los golden fixtures. El trabajo propuesto abarca el esquema del descriptor, el resolver de task-start, las allowlists de Gateway y Cedar, la validación de publicación y los fixtures; se considera terminado cuando estas piezas admiten una configuración dinámica del entorno validada.

Escrito por el modelo de indexación a partir del texto del issue.