feat: add prepush lifecycle hook (#498)

* feat: add prepush lifecycle hook

Add `prepushScript` configuration — an optional shell script that runs
automatically before pushing code to the remote repository. Reuses the
existing `executeLifecycleHook` infrastructure (bash execution, 2-min
timeout, error propagation on non-zero exit). When unconfigured the
hook is a no-op.

Made-with: Cursor

* fix: add prepushScript to run-context API response, fix UI separator

Include prepushScript in the settings returned by the run-context
endpoint so the hook actually fires in production. Also fix the
separator pattern in AgentSettings to match the existing convention
(spacer + hr + spacer instead of margin).

Made-with: Cursor

main.ts

@@ -275,6 +275,7 @@ export async function main(): Promise<MainResult> {
       apiToken: runContext.apiToken,
       modes,
       postCheckoutScript: runContext.repoSettings.postCheckoutScript,
+      prepushScript: runContext.repoSettings.prepushScript,
       prApproveEnabled: runContext.repoSettings.prApproveEnabled,
       modeInstructions: runContext.repoSettings.modeInstructions,
       toolState,