refactor(runner): clarify the session event-demux (rename env2, extract helpers)

Readability-only, no behavior change. Renames the SessionEnvironment `env2` to
`environment` and extracts the two session-lifetime listeners into documented
`routeSessionEventToActiveTurn` / `routePermissionRequestToActiveTurn` helpers, so
`acquireEnvironment` reads as clear steps and the demux data flow is obvious.
Addresses Mahmoud's #5156 review note that the event handling was hard to read.
723 tests unchanged, typecheck clean, reviewer confirmed no semantic drift.

Lands on the stacked approvals lane (top of the keep-alive stack) because the working
tree is the stack's applied state and GitButler attributes the edit to the top lane;
it rides into big-agents with the stack.

Claude-Session: https://claude.ai/code/session_01AumZJ9xRd4XYNHThqy4rTv

Author: mmabrouk