feat: add counterscarp --clean to free local disk space

[RunTimeAdmin]

Summary

Adds a manual disk cleanup command for Counterscarp/ScarpShield deployments where scan artifacts accumulate locally (reports/, uploads/, results/, .scarpshield/ state files, etc.).

Changes

• New cleanup.py module with shared housekeeping logic and retention defaults aligned with existing docs
• counterscarp --clean and counterscarp --clean --dry-run CLI flags (no --target required)
• Webapp startup cleanup now uses the same module and also purges stale results/ directories (30 days), which were previously only documented via manual find commands
• Tests and documentation updates in docs/CONFIGURATION.md and docs/DEPLOYMENT.md

Usage

cd /path/to/counterscarp-engine
counterscarp --clean --dry-run   # preview reclaimable space
counterscarp --clean             # remove stale artifacts

Retention (unchanged policy)

Data	Retention
State / cache (.scarpshield/, legacy .counterscarp/)	30 days
reports/	90 days
uploads/	7 days
results/	30 days