test-assistants.sh: emit stream-json from Antigravity too

Adds --output-format stream-json to the agy launch, matching Claude Code and
Cursor, so all five assistants share one output shape through unwrap_log().
NOT yet verified: Antigravity's weekly quota is exhausted, so its report can't
be exercised in a run. The flag choice is low-risk — stream-json is the same
format Claude Code and Cursor already emit and parse — but this leg stays
unverified until a run can confirm it. PR remains draft until then.

Author: mraible

test-assistants.sh

@@ -458,7 +458,7 @@ ASSISTANTS=(
   "Codex|codex|~/.agents/skills|exec %%PROMPT%% --skip-git-repo-check --json"
   "Copilot CLI|copilot|--plugin-dir|-p %%PROMPT%% --plugin-dir $REPO --allow-all --output-format json"
   "Cursor|agent|--plugin-dir|-p %%PROMPT%% --plugin-dir $REPO --force --trust --output-format stream-json"
-  "Antigravity CLI|agy|~/.agents/skills|-p %%PROMPT%% --dangerously-skip-permissions"
+  "Antigravity CLI|agy|~/.agents/skills|-p %%PROMPT%% --dangerously-skip-permissions --output-format stream-json"
 )
 
 want() {