[codex] Coerce stringified spawn agent params #667

Job	Run time
build-freebuff	1m 23s
build-and-smoke-freebuff-windows	2m 25s
e2e-help-command	1m 18s
e2e-slash-commands	52s
e2e-agent-startup	2m 15s
e2e-version	1m 1s
e2e-startup	1m 23s
e2e-code-edit	52s
e2e-ads-behavior	1m 1s
e2e-terminal-command	1m 15s
	13m 45s

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