Make bare kitaru login target the managed cloud via device flow #966

Problem

kitaru login with no arguments errors: "Login requires a SERVER argument or --local." The two real choices a new user has are a local server in Docker or the managed cloud, and only the first has a command. Signing in to the managed cloud means knowing your workspace URL first and typing kitaru login <url>.

This blocks the one-line installer's closing message (#965), which today has to say:

kitaru login --local       local, in Docker. Free, open source.
https://cloud.kitaru.ai    managed cloud. 14-day trial, no credit card required.
                           then: kitaru login <your workspace URL>

Expected behavior

• kitaru login (bare) starts the device-auth flow against the managed cloud, resolves the user's workspace from the account, and stores the credential, so the second line above becomes plain kitaru login.
• kitaru login --local and kitaru login <url> unchanged.
• In a terminal, bare kitaru login may first show a three-line chooser (local Docker, managed cloud, other URL) with local highlighted; without a terminal it should go straight to the cloud device flow or fail with the same hint as today.
• Wording in the CLI: "managed cloud", not a product name; mention the 14-day trial, no credit card required.

Related: #953 (kitaru setup), #952 and #965 (installer).