Improve invalid config JSON error message with file path

[404-Page-Found]

Task summary

Improve the error shown when config.json contains invalid JSON so users see a clear message that includes the config path and a hint on how to fix it.

Why this is a good first issue

This is a small UX improvement in one code path and does not require major architecture changes.

Suggested files or areas

• src/config/store.ts
• Any command paths that call loadConfig() / loadOrPromptConfig()
• Tests covering malformed JSON handling

Acceptance criteria

• Catch JSON parse failures from loadConfig() and return/throw a user-friendly error.
• Include the config file path in the message.
• Keep non-JSON errors unchanged unless necessary.
• Add a regression test for malformed JSON input.

Extra context

Current behavior bubbles raw parse errors, which are technically correct but not very actionable for CLI users.

[nightcityblade]

Hi, I'd like to work on this. I'll submit a PR shortly.