Defer environment selection changes until the next turn - #46310

## Why

Updating environment selections while a turn is running must not redirect its tools or prevent its pending environment setup from completing.

## What changed

- Store future environment selections separately from active environments, and activate them when preparing new work with no task running.
- Keep turn contexts and MCP workspace roots tied to the active environment snapshot. Report saved selections through thread settings and `environment_selections()`.
- Route environment configuration and failure callbacks to matching active or future selections so each can finish setup independently.
- Stop the running task before manual compaction adopts the saved environments.

## Testing

Add regression tests that preserve a running task's working directory and workspace roots, then adopt the saved selection for the next task. Add remote environment coverage showing that future setup can complete while the active turn waits, with each turn receiving its own capability roots.

GitOrigin-RevId: 09a6b90131d2ab2441ce2fe9b0368a3668889f31