feat: Replace hardcoded paths in simulate_stream.py with env vars

[exploring-curiosity]

Closes #19

What changed

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).

Approach

Read simulate_stream.py to identify all hardcoded paths starting with /mnt/persist/. Replace each hardcoded path with an os.environ.get() call using a descriptive environment variable name (e.g., STREAM_INPUT_DIR) and a sensible local default like './tmp/stream_input'. Ensure the environment variable names are consistent and documented if necessary.
Files: data_pipeline/online_pipeline/simulate_stream.py

Verification

Tier	Result	Detail
T0 Diff sanity	pass	internal: diff sanity
T1 Static	skipped	project environment unavailable: dependency install failed: To find more informa

Why this is a draft

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

Note: T1 (project environment unavailable: dependency install failed: To find more information, look for "package discovery" on setuptools docs. hint: This usually indicates a problem with the package or the build environment.) - these were not verified.