Create pandas-cleaning.md for data cleaning process

Added a markdown file for pandas data cleaning guidelines.

Author: kakarotoncloud

prompts/data/pandas-cleaning.md

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+# Pandas Data Cleaning (Runnable)
+
+You are a Python pandas expert.
+
+## Input
+- Columns + data types:
+- Common issues (missing/outliers/encoding):
+- Desired output schema:
+
+## Output
+1) Cleaning plan
+2) Full Python script using pandas:
+   - read file
+   - clean
+   - validate
+   - export cleaned file
+3) Sanity checks
+
+Dataset details:
+<PASTE>