Feature Request: discoverable and configurable execution environment capacity

## Problem

Sessions that run in a hosted environment receive a fixed allocation of CPU, memory, and disk. That allocation is not visible before the session starts and is not configurable. For a large repository, the first indication that capacity is insufficient is a failure partway through dependency installation, a build, or a test run — after the setup time has already been spent.

Large repositories, native application builds, and multi-platform validation all need substantially more capacity than a small allocation provides. There is also no documented pattern for delegating validation that the hosted environment cannot perform to external CI and consuming the result.

## What is missing

- Effective CPU, memory, disk, operating system, and timeout values are not reported to the caller.
- There is no way to select a larger or differently configured environment where one is available.
- Caching behavior across sessions is undocumented.
- There is no documented pattern for delegating unsupported platform validation to external CI.

## Proposed behavior

- The session reports its effective environment — operating system, CPU count, memory, available disk, timeout, and cache configuration — before work begins.
- Repository or organization owners can select from approved environment profiles.
- Documented guidance exists for delegating validation that cannot run in the hosted environment to external CI, and for the session to consume those results.
- Capacity-related failures are identified as such, rather than surfacing as generic build or test errors.

## Example scenario

A caller starts a session against a large repository. The session reports its capacity up front, so the caller knows immediately whether a full build is viable or whether validation should be delegated to external CI.

## Acceptance criteria

- Effective environment capacity is programmatically discoverable.
- At least one mechanism exists to request a larger environment where supported.
- Capacity exhaustion produces a distinct, documented error.

## Related

- #1223 — sandbox support, which overlaps on lifecycle, providers, and resource configuration.

No files, tests, or concrete entry points are named. Start by tracing the hosted-session lifecycle and the providers and resource configuration discussed in related issue #1223. Done means the acceptance criteria are addressed: discoverable capacity, a supported larger-environment mechanism, and distinct documented capacity errors.

Written by the indexing model from the issue text.