docs: document runner failure session retention

Co-Authored-By: Oz <oz-agent@warp.dev>

Author: warp-agent-staging[bot]

src/content/docs/factories/factory-as-code.mdx

@@ -449,6 +449,7 @@ Optional. Each file defines a runner: the compute a run executes on. The runner'
 description: Linux runner for payments builds and tests
 setupCommands:
   - corepack enable
+failureSessionRetentionMinutes: 15
 instanceShape:
   vcpus: 4
   memoryGb: 8
@@ -467,6 +468,12 @@ Optional. Shell commands run in order while the sandbox is prepared.
 
 Optional. The compute size, as `vcpus` and `memoryGb`. Omit it to take the workspace default.
 
+### `failureSessionRetentionMinutes`
+
+Optional. An integer from `1` through `60` that keeps a failed run's session open for inspection. Activity in the session restarts this idle window. There is no runner-level default: the runner value overrides the run environment's setting, while an omitted value uses the environment's setting or tears down the session immediately when neither is set.
+
+The value must not exceed the maximum runtime available to the sandbox. Warp disables retention for a run when the value exceeds that limit, and the sandbox's runtime limit can end the session before the full idle window elapses. Secrets remain in the sandbox, and compute credits may apply while the session stays open.
+
 ### `platform`
 
 The operating system and architecture. `os` is `linux` (the default) or `macos`, and `arch` is `x86_64` (the default on Linux) or `aarch64` (the only option on macOS). Linux runners require `linux.dockerImage`, the container image the sandbox boots, so every Linux runner declares a `platform` section. macOS runners accept an optional `mac.version` (`"14"`, `"15"`, `"26"`, or `"27"`; quote it, and it defaults to `"26"`).