fix: replace md5 with sha256 for FIPS-compliant cache key generation

[octo-patch]

Fixes #701

Problem

ShellGPT fails immediately on FIPS-enabled systems because the cache uses MD5 to generate cache keys. MD5 is not approved for use on FIPS 140-2/140-3 compliant systems, causing a ValueError when hashlib.md5() is called.

Solution

Replace md5 with sha256 for cache key generation. SHA-256 is FIPS-compliant and serves the same purpose here — producing a deterministic, fixed-length key from the request parameters. This is a non-cryptographic use case (cache key deduplication), so SHA-256 is a drop-in replacement with no functional difference.

Note: Existing cache files (named with MD5 hashes) will not be matched after this change, so the cache will be cold on first run. Old files will be gradually evicted by the existing _delete_oldest_files cleanup logic.

Testing

• Verified that the cache key generation works correctly with SHA-256
• The change is minimal and contained to sgpt/cache.py