Track disposable sandbox principal, runtime tools, and package import contracts

## Problem
WP Codebox currently compensates for generic agent-substrate gaps when running agents inside disposable WordPress Playground sandboxes:

- bypasses `agents/chat` permission filters inside the browser runner
- injects sandbox runtime tools through Data Machine's `datamachine_resolved_tools` filter
- imports Data Machine agent bundles as the practical transport for sandbox agent definitions
- depends on Data Machine-specific tool names/policy vocabulary for sandbox-safe vs parent-only tools
- relies on product/Data Machine pending-action adapters where generic approval substrate should be enough

These are reasonable local adaptations today, but the generic substrate should define enough contracts that Codebox can be a clean sandbox runtime consumer rather than a Data Machine-specific adapter.

Related Codebox pressure points:
- browser runner permission bypass in disposable Playground scope
- runtime `filesystem-write` tool registration
- sandbox Data Machine tool allow/deny policy
- agent bundle import path
- pending-action/apply-back integration

## Desired direction
Agents API should provide generic contracts for:

- a trusted disposable sandbox principal / runtime execution scope
- runtime-local tool source registration and mediation
- package or agent-definition import usable by ephemeral runtimes
- scoped tool capability metadata, e.g. safe in disposable runtime vs parent control plane only
- pending-action mediation hooks usable by products without direct Data Machine helper dependencies

## Acceptance criteria
- Define which pieces already exist in Agents API and which are missing.
- Propose the smallest substrate additions needed for WP Codebox to remove Data Machine-specific compensation.
- Keep concrete storage, UI, workflow jobs, and product apply behavior outside Agents API.
- Link follow-up implementation issues in WP Codebox/Data Machine where consumer migrations are needed.

## Source evidence
- WP Codebox `trait-wp-codebox-abilities-browser-runner.php`
- WP Codebox `class-wp-codebox-agent-sandbox-runner.php`
- WP Codebox `sandbox-datamachine-tool-policy.*`
- WP Codebox `class-wp-codebox-data-machine-pending-actions.php`