[codex] Coerce stringified spawn agent params (#658) #668

Job	Run time
build-and-smoke-freebuff-windows	2m 42s
build-freebuff	1m 13s
e2e-version	1m 9s
e2e-terminal-command	1m 15s
e2e-agent-startup	2m 23s
e2e-slash-commands	52s
e2e-ads-behavior	1m 11s
e2e-code-edit	54s
e2e-help-command	1m 37s
e2e-startup	1m 2s
	14m 18s

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