## What changed
- Remove `subagent_thread_environments` and the separate configured environment bindings. Spawned agents inherit the invoking step's environment snapshot.
- Retain environment selections across child eviction without retaining executor bindings. Reload V2 children against the parent's captured environments, requiring matching environment IDs, working directories, and workspace roots while preserving permission validation.
GitOrigin-RevId: d820c63ea20f5834ed88071d16c60683f2545406
## Why
Temporary environment attachments for a parent's turn should not become a child's persistent environment configuration or survive child eviction.
## What changed
- Add the default-disabled `subagent_thread_environments` feature flag. When enabled, subagents inherit configured thread environments captured with the spawning step instead of active turn selections.
- Track configured bindings separately from turn attachments, preserving pending configuration ownership when turn selections change.
- Retain configured bindings across child eviction and reload, excluding temporary attachments. Preserve the child's workspace when the parent switches workspaces, while enforcing current roots and permissions when the parent still configures the same workspace.
## Testing
Extend coverage for both multi-agent versions, forked children, empty configured environments, pending configuration, and legacy inheritance. Add eviction and reload cases covering temporary attachments, workspace changes, deferred executor startup, and permission validation.
GitOrigin-RevId: 4533645503d5f863a9ccc064386ed7ca169d98b7
## What changed
Move child configuration, task path construction, fork usage hints, and backend spawning into `core::thread_spawn`. Add request, result, and error types that distinguish preparation failures from backend errors.
Route the v2 `spawn_agent` handler through the shared helper while retaining tool-specific telemetry, activity events, and response formatting in the handler.
GitOrigin-RevId: a09a8f66beb41c1403220750d236a12717dbfc6c
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