fix: resolve flake8 failures breaking the Conda workflow

[romeokienzler]

Problem

The Python Package using Conda workflow fails on every push. Its flake8 hard-fail gate (--select=E9,F63,F7,F82, which stops the build on syntax errors and undefined names) flagged a mix of real bugs and template files:

operator_utils.py: F821 undefined name 'time' / 're' / 'subprocess'
fibonacci.py:       F821 undefined name 'os'
cosutils.py:        F821 undefined name 'CHUNK_SIZE'
s3_kv_store.py:     F821 undefined name 'MLX'
c3/templates/*:     E999 SyntaxError: invalid syntax  (x6)

Fixes

Real bugs (these would crash at runtime, not just lint):

• operator_utils.py — run_and_log() uses time, re, subprocess with no imports → add them.
• fibonacci.py — uses os.getenv with no import os → add it.
• cosutils.py — _download() reads with an undefined CHUNK_SIZE → use the module's existing MIN_CHUNK_SIZE (8 MiB), matching _upload.
• s3_kv_store.py — main() instantiates MLX(...) but the class is S3KVStore → fix the name.

Not a bug — false positive:

• c3/templates/* — these are string.Template code-generation files with ${placeholder} substitutions; they are intentionally not valid standalone Python. Excluded via extend-exclude in setup.cfg [flake8] so the exclusion applies wherever flake8 runs.

Also removed two local run artifacts (experiment_logs/, iterate_study.db) that slipped in, and added them to .gitignore.

Verification

$ flake8 src terratorch_iterate --count --select=E9,F63,F7,F82
0

All four edited modules parse (AST) and operator_utils imports cleanly.

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