Make `--input-store` errors user-friendly, including relative paths and corrupt JSON

[httpdss]

Summary

The input store implementation assumes --input-store has a directory component and valid JSON. Relative filenames such as --input-store input.json, unreadable paths, or corrupt JSON can currently produce raw Python exceptions during generation.

Evidence

Observed locally:

python -m structkit.main generate --non-interactive --input-store input.json simple.yaml out
Traceback ...
FileNotFoundError: [Errno 2] No such file or directory: ''

Relevant code:

• structkit/input_store.py:12-14 calls os.makedirs(os.path.dirname(input_file)); for input.json, os.path.dirname(...) is ''.
• InputStore.load directly calls json.load, so corrupt JSON would raise json.JSONDecodeError as a traceback.
• These errors surface from TemplateRenderer/FileItem construction rather than a clean CLI message.

Proposed fix

• Treat an empty dirname as current directory and avoid os.makedirs('').
• Catch json.JSONDecodeError and OSError with a clean InputStoreError message.
• Catch that domain error in commands/tools that construct TemplateRenderer.
• Include the input-store path in error messages.

Acceptance criteria

• --input-store input.json works and creates/uses ./input.json.
• Corrupt input-store JSON exits 1 with a clean message and no Traceback.
• Permission/read/write errors for the input store include the path and exit 1 cleanly.
• Tests cover relative path, corrupt JSON, and unwritable/read-error path where practical.