Replace hardcoded paths in simulate_stream.py with env vars

[exploring-curiosity]

Paths like /mnt/persist/... are hardcoded, making the script non-portable and impossible to run locally. Replace all hardcoded paths with configurable environment variables with sensible local defaults (e.g., ./tmp/stream_input).

[exploring-curiosity]

Could not complete this yet.

Reason: nothing beyond diff sanity was verified - refusing to merge unverified work

The attempted work is preserved in draft PR #20.

Leaving this open - the problem is still real, it just needs a different approach.