Feature: Add environment variable support for additional CLI arguments #123

Description

Follow-up to #122. In addition to --structures-path, several other CLI arguments would benefit from environment variable support to improve usability, especially in CI/CD pipelines and containerized environments.

Proposed Environment Variables

High Priority

1. STRUCTKIT_GLOBAL_SYSTEM_PROMPT (for --global-system-prompt / -p)

   • Typically a long or sensitive value that users want to set once
   • Avoids repeating the same prompt across multiple invocations
   • Ideal for CI/CD workflows and container initialization

2. STRUCTKIT_INPUT_STORE (for --input-store / -n)

   • Allows users to set a consistent default location for input data
   • Useful for workflows that need persistent input across multiple runs
   • Currently defaults to /tmp/structkit/input.json

3. STRUCTKIT_BACKUP_PATH (for --backup / -b)

   • Set a default backup location project-wide or environment-wide
   • Saves typing in repetitive operations
   • Useful for ensuring backups go to a specific location (e.g., mounted volume in containers)

Medium Priority

4. STRUCTKIT_FILE_STRATEGY (for --file-strategy / -f)

   • Let users set a preferred default strategy (overwrite, skip, append, rename, backup)
   • Could prevent accidental data loss if set to 'skip' or 'backup' by default

5. STRUCTKIT_NON_INTERACTIVE (for --non-interactive)

   • Boolean flag useful for CI/CD pipelines
   • Could be set in environment and applied across all commands

6. STRUCTKIT_OUTPUT_MODE (for --output / -o)

   • Some users might prefer 'console' output by default
   • Useful for pipeline integration

Implementation Notes

• Command-line arguments should take precedence over environment variables
• The pattern from STRUCTKIT_LOG_LEVEL in structkit/main.py (lines 72-74) can serve as a reference
• Consider adding environment variable hints to the help text for each argument

Priority Recommendation

Start with High Priority items (#1-3) as they provide the most value for typical use cases.