Feature: Support setting structures-path via environment variable

[httpdss]

Description

Currently, the --structures-path (-s) argument can only be set via the command-line flag. It would be convenient to also support setting this via an environment variable, similar to how STRUCTKIT_LOG_LEVEL works for logging.

Motivation

• Allows users to set a default custom structures path without repeating the -s flag on every command
• Useful for CI/CD pipelines and Docker containers where environment variables are the standard way to configure applications
• Consistent with the existing pattern used for STRUCTKIT_LOG_LEVEL

Proposed Solution

Add support for a STRUCTKIT_STRUCTURES_PATH environment variable that would be used as the default value for --structures-path if not explicitly provided via the command-line flag.

Example Usage

export STRUCTKIT_STRUCTURES_PATH=~/custom-structures
structkit generate python-basic ./my-project
# Would behave the same as:
# structkit generate -s ~/custom-structures python-basic ./my-project

Priority

Command-line arguments should take precedence over the environment variable (current behavior for other similar tools).

Related Code

The environment variable pattern is already implemented for logging in structkit/main.py (lines 72-74), which could serve as a reference for this implementation.