ROBOCO_TOOLCHAIN_MATCH_ENABLED (default-off) gates the whole subsystem.
Adds work_sessions.toolchain_python / toolchain_status (nullable VARCHAR(20))
recording the interpreter the workspace was provisioned with and whether the
target's suite can be executed (ok | broken | unknown). Migration 042 verified
with a real upgrade head + downgrade -1 + re-upgrade on a throwaway Postgres.
