Optional LLM key persistence for `blink init`

[blinkagent]

Problem

Users have to manually locate and enter their LLM key every time they run blink init for a new agent, which creates friction in the workflow.

Proposal

Add an optional feature to remember LLM keys between blink init sessions:

1. Timing: Only show the prompt on the second blink init (avoid making first-time experience longer)
2. Dialog: After user inputs their LLM key, ask: "Remember the key for the next blink init?"
3. Transparency: If confirmed, print: "Saved the key in ~/.blink/keys.txt"
4. Security consideration: User needs to be clearly aware the key is stored persistently in their home directory in plain text

Security Trade-offs

Currently keys are only stored in .env.local within each project, making it clear where to delete them. Storing in a global location like ~/.blink/keys.txt introduces risk if the file leaks. The feature should prioritize transparency about where keys are stored and how to remove them.

Priority

Nice-to-have / backlog item