Use sprite login instead of sprite org auth to authenticate the CLI

[mlr]

The quickstart and the CLI authentication page tell new users to run sprite org auth. On a machine that has never run sprite login, it fails:

Error: no Fly.io token found. Please run 'sprite login' first

This happens even when flyctl is logged in, because sprite org auth doesn't read the flyctl session. The fallback the docs suggest (fly auth logout, fly auth login, then retry) doesn't fix it either.

This PR switches first-time setup and re-authentication to sprite login:

• quickstart.mdx: the Authenticate step uses sprite login. The flyctl fallback note is gone.
• cli/authentication.mdx: Quick Setup, Authentication Flow and Troubleshooting use sprite login. The claim that the CLI reuses the flyctl session is gone, and "Adding Organizations" now says to log in first.
• working-with-sprites.mdx: the connection-error tip uses sprite login.

Aligns with the agent skill

The Sprites agent skill at https://fly.io/agent-skills/sprites-quickstart/SKILL.md already uses sprite login, and so do the agent-readable pages at fly.io/sprites. With this change, the docs give people the same instructions that agents already get.

The CLI agrees too. sprite org --help says: "After logging in with sprite login, use org to manage your organization tokens and defaults."

Unchanged

• sprite org auth --org my-team in the multiple-organization sections. That's a correct use: it adds a token for another org after sprite login.
• cli/commands.mdx, which is generated from the CLI help.

Testing

I ran the quickstart with coding agents against a Fly.io account that had never used Sprites:

Run	Auth step	Result
Claude Code	sprite login	40 s, 10 tool calls, 1 browser approval
Claude Code	sprite org auth	Failed with the error above, then fell back to sprite login: about 90 s, 22 tool calls, 2 browser approvals plus a pasted code
opencode	sprite login	7 tool calls, 1 browser approval
opencode	sprite org auth	Failed with the error above, then fell back to sprite login: 11 tool calls

Every sprite org auth run ended up on sprite login anyway.

[kcmartin]

Reproduced the error with an empty config: sprite org auth prints "no Fly.io token found. Please run 'sprite login' first". sprite login -o matches the CLI help, and Vale is clean on the changed lines.

Optional, since you're already reworking this page: under "Select an Organization", "To switch organizations: sprite org auth --org my-team" actually adds a token for another org rather than switching. "To add another organization:" would be more accurate.