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

Beginne mit ROADMAP.md und docs/design/COMPUTE.md und untersuche anschließend den Registry-Publish-Pfad sowie die unter contracts/ oder cdk/test/ genannten Orte für Golden Fixtures. Die vorgeschlagene Arbeit umfasst das Descriptor-Schema, den task-start-Resolver, Gateway- und Cedar-Allowlisten, die Publish-Validierung und Fixtures; abgeschlossen ist sie, wenn diese Komponenten eine validierte dynamische Umgebungskonfiguration unterstützen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.