refactor: migrate API key storage from API_KEY.txt to activity_root/data

[adarshkumar23]

Summary

Replaces the flat-file API_KEY.txt approach with Sugar's standard activity_root/data/ directory for persistent per-activity configuration, as discussed by @mebinthattil in #60.

Problem

The current code reads the API key from API_KEY.txt in the bundle's working directory. This has several issues:

1. Risk of accidental commits — the key file sits alongside source code
2. Not Sugar-standard — Sugar activities should use get_activity_root()/data/ for persistent configuration
3. Unreliable path — the working directory may not be where the bundle lives depending on how the activity is launched

Changes

• LLM.py: Read/write the API key from get_activity_root()/data/api_key instead of API_KEY.txt
• LLM.py: Add save_api_key() public helper for programmatic key persistence (enables future settings UI)
• LLM.py: Improve error messages to log the expected key file path
• activity.py: Export save_api_key from the LLM import
• README.md: Add "API Key Configuration" section documenting the new storage location

How it works

The data/ subdirectory of get_activity_root() survives across activity invocations and is the Sugar-standard location for per-activity persistent state. This is the same pattern already used in chat.py (line 249).

If no key is found, the activity logs the expected path and gracefully falls back to the on-device SLM or AIML brain — no crash.

Related

• Addresses feedback from @mebinthattil in chore: update .gitignore to prevent API key leaks #60
• Resolves the original TODO comment: #TODO: Dont hard code these, need to see how sugar as a whole manages API Keys

[mebinthattil]

Thank you for these changes, would love to see a screen recording of these changes in action inside sugar / screenshots of these changes working.

Also include info on how to request for an API key from Sugar-AI. Check the Sugar-AI docs for more context.

[mebinthattil]

No newline in EOF. Diff your changes before pushing from next time, you'll be able to catch these.

[adarshkumar23]

Fixed in 57cb868. Here's the diff confirming the change:

Also noted — will run git diff --staged before every commit going forward.

Fixing.EOF.Line.in.LLM.py.and.Review.Comments.mp4

[mebinthattil]

I was asking about a screen recording of you running these changes inside the activity and not just running LLM.py in your terminal.
Please send that video. Be sure to test it inside of sugar.