Session vars (HERMES_SESSION_*) have a process-global os.environ mirror written
last-writer-wins as a CLI/cron fallback and never cleared. Under a concurrent
multi-session host (messaging gateway, ACP adapter, API server, TUI) that global
belongs to whichever turn wrote it last. A subprocess spawned from a task whose
session ContextVar is _UNSET (a sibling task that never bound, or one that
inherited another session's context) inherited the FOREIGN global and acted on
another session's identity.
Add a session_context_engaged() latch (set once any host calls set_session_vars)
and route both terminal spawn paths through a single _inject_session_context_env
chokepoint: once engaged, a bound ContextVar (incl. "") is authoritative and an
_UNSET var is STRIPPED rather than inheriting the possibly-foreign global. Pure
single-process CLI/one-shot (never engaged) keeps the inherited fallback.
Salvaged from #50531 (supersedes #49922). local.py hunk re-applied by intent
onto the current hermes_subprocess_env refactor.
Co-authored-by: PolyphonyRequiem <3107779+PolyphonyRequiem@users.noreply.github.com>
