Add flow config extract-key command

[chasefleming]

Adds a new command to extract inline private keys from flow.json into separate
.pkey files for improved security.

Usage

flow config extract-key <account-name>  # Extract a specific account
flow config extract-key --all           # Extract all accounts with inline keys
flow config extract-key                 # Interactive selection

What it does

• Converts accounts from inline hex key format
  ("key": "0xdeadbeef...")
  to file-based format
  ("key": { "type": "file", "location": "./account.pkey" })

• Automatically adds key files to .gitignore and .cursorignore

• Prevents accidental overwrites if a key file already exists

---

For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[codecov-commenter]

Codecov Report

❌ Patch coverage is 68.05556% with 23 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/config/extract-key.go	67.60%	17 Missing and 6 partials ⚠️

📢 Thoughts on this report? Let us know!