Use -- separator in sprite exec examples

[kcmartin]

Summary

Updates sprite exec examples in the public docs to use the -- separator between sprite's own flags and the command's args.

Reported as #164. Examples like sprite exec ls -la fail with unknown shorthand flag: 'l' in -la because cobra's flag parser keeps consuming flags after the positional command. The fix is sprite exec -- ls -la. This matches the convention adopted upstream (the sprite exec --help examples and usage line were updated to show -- in the next rc).

Changes

• quickstart.mdx: every sprite exec example uses --.
• working-with-sprites.mdx: every sprite exec example uses --. Deprecated -tty shorthand swapped to --tty -- <cmd>.

Tests

Tested all commands against a live Sprite running rc43 version of the CLI

Closes #164.
Closes #180 (duplicate report, same root cause).

[github-actions]

Preview Deployment

Name	URL
Preview	https://pr-182-superfly-sprites-docs.fly.dev

Commit: f99e5ab

[github-actions]

Lighthouse Results

URL	Performance	Accessibility	Best Practices	SEO
/	🟠 80	🟢 100	🟢 96	🟢 100
/quickstart/	🟢 91	🟢 100	🟢 100	🟢 100
/working-with-sprites/	🟢 94	🟢 100	🟢 100	🟢 100

[github-actions]

E2E Test Results

✅ Tests success

Ran against: https://pr-182-superfly-sprites-docs.fly.dev

[kcmartin]

@jbellerb This is a companion PR to your https://github.com/superfly/sprite-env/pull/436
Let me know if any changes are needed.