Preserve pending environments when spawning subagents

[copyberry]

Preserve pending environments when spawning subagents

Why

Spawning a subagent before an environment was ready dropped the starting environment from the child's selections. The child needs to retain that environment and receive its configuration or failure when preparation finishes.

What changed

• Pass the spawning step's environment snapshot to children, preserving ready and starting environments and configuration ownership.
• Propagate the original owner's first configuration result through descendants, applying each session's validation and leaving independently configured children unchanged.
• Preserve pending configuration waits across executor retries. Later owner updates do not overwrite the child's inherited configuration.

Testing

Add regression coverage for fresh and forked subagents, readiness through turn-settings updates, failure propagation to children and grandchildren, child configuration precedence, and executor retries. Add a request snapshot showing a child using wait_for_environment before execution tools become available.