fix(agents): pass Claude system prompt via file

docs/.generated/config-baseline.sha256

@@ -1,4 +1,4 @@
-dae9ece3ac683a0bed2835d96d4373f65ab955b8b901df0bcdeedc565ade6ed6  config-baseline.json
-7cd52f77b1e0ecb50d2119b4c21d6d51d336a0c752a44cbaf8df1efa9ef538c0  config-baseline.core.json
-d72032762ab46b99480b57deb81130a0ab5b1401189cfbaf4f7fef4a063a7f6c  config-baseline.channel.json
-0504c4f38d4c753fffeb465c93540d829df6b0fcef921eb0e2226ac16bdbbe07  config-baseline.plugin.json
+445663bd6907368befbfd76f6fcc58f9dc282244697f44e9860391e51e6f2f83  config-baseline.json
+f54f808dc85123a5ba788618a6dff7f2c869ced639dd0db34a86802985730dc6  config-baseline.core.json
+7cd9c908f066c143eab2a201efbc9640f483ab28bba92ddeca1d18cc2b528bc3  config-baseline.channel.json
+7825b56a5b3fcdbe2e09ef8fe5d9f12ac3598435afebe20413051e45b0d1968e  config-baseline.plugin.json

docs/gateway/cli-backends.md

@@ -122,6 +122,8 @@ The provider id becomes the left side of your model ref:
           sessionMode: "existing",
           sessionIdFields: ["session_id", "conversation_id"],
           systemPromptArg: "--system",
+          // For CLIs with a dedicated prompt-file flag:
+          // systemPromptFileArg: "--system-file",
           // Codex-style CLIs can point at a prompt file instead:
           // systemPromptFileConfigArg: "-c",
           // systemPromptFileConfigKey: "model_instructions_file",

docs/gateway/config-agents.md

@@ -443,6 +443,7 @@ Optional CLI backends for text-only fallback runs (no tool calls). Useful as a b
           sessionArg: "--session",
           sessionMode: "existing",
           systemPromptArg: "--system",
+          // Or use systemPromptFileArg when the CLI accepts a prompt file flag.
           systemPromptWhen: "first",
           imageArg: "--image",
           imageMode: "repeat",