🐛 bug fix: adopt agy onboarding wizard relay support - #5753

`agy` is a first-class backend throughout this file (launch flags, --effort
handling, capability reporting) but getCLIState() has no branch for it, so an
Antigravity pane always fell through to 'unknown'. The relay then waited out
CLI_READY_TIMEOUT_MS and handed the task back — with the CLI sitting on a
dismissible dialog the whole time.

Antigravity gates first run behind a three-step wizard: colour scheme -> Terms
of Service -> folder trust. This is not a once-per-image cost: agents that
share a $HOME symlink one ~/.gemini, and whenever one writes
antigravity-cli/cache/onboarding.json mode 600 every OTHER agent gets EACCES
and re-enters the wizard from the start.

Two changes:

- getCLIState() gains an `agy` branch mapping the wizard to 'onboarding', the
  "not signed in / Select login method" screen to 'needs-login', and the
  banner/prompt to 'ready'.

- blockingPromptKey() learns the Terms of Service step. It is the one page a
  bare Enter cannot leave: focus sits on the consent CHECKBOX, where Enter
  toggles rather than advances ("enter Toggle"), so the relay would dismiss in
  a loop until timeout. Down moves to the button row and Right selects [Done];
  the caller appends Enter. The colour-scheme and folder-trust steps DO advance
  on a bare Enter and deliberately return null.

Verified against real captured panes from a live fleet: all five agy states
classify correctly, and codex's existing prompts are unaffected.

Signed-off-by: James Reilly <jreilly1821@gmail.com>