Optional LLM key persistence for `blink init`

## 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