fix: handle undefined HOME environment variable in key store resolution

[Tyagiquamar]

Resolves #286

Description

When running pyth, pyth_admin, or pythd in environments where the HOME environment variable is undefined (such as minimal Docker containers, AWS Nitro enclaves, or systemd services with DynamicUser=true), attempting to construct std::string(getenv("HOME")) resulted in std::logic_error: basic_string::_M_construct null not valid and aborted the process.

Changes

1. Added key_store::get_default_dir() helper method that safely inspects getenv("HOME"), falling back to ./.pythd/ when HOME is undefined.
2. Updated pcapps/pyth.cpp and pcapps/pyth_admin.cpp to evaluate default key directory dynamically instead of initializing static global std::string constants with unchecked getenv("HOME").
3. Updated pcapps/pythd.cpp get_key_store() to use key_store::get_default_dir().
4. Added unit test in pctest/test_unit.cpp to verify key_store::get_default_dir() handles both defined and undefined HOME environment variables without throwing.

Verification

• Built all targets (pc, pyth, pyth_admin, pythd, test_unit) in Linux Docker environment.
• Executed unit tests (test_unit) with all checks passing.
• Verified CLI help commands (pyth -h, pyth_admin -h, pythd -h) execute cleanly with HOME undefined.