fix(cli/run): properly serialize error objects to prevent [object Object] output

- Add serializeError utility to handle Error instances, plain objects, and nested message paths
- Fix handleSessionError to use serializeError instead of naive String() conversion
- Fix runner.ts catch block to use serializeError for detailed error messages
- Add session.error case to logEventVerbose for better error visibility
- Add comprehensive tests for serializeError function

Fixes error logging in sisyphus-agent workflow where errors were displayed as '[object Object]'

src/cli/run/runner.ts

@@ -2,7 +2,7 @@ import { createOpencode } from "@opencode-ai/sdk"
 import pc from "picocolors"
 import type { RunOptions, RunContext } from "./types"
 import { checkCompletionConditions } from "./completion"
-import { createEventState, processEvents } from "./events"
+import { createEventState, processEvents, serializeError } from "./events"
 
 const POLL_INTERVAL_MS = 500
 const DEFAULT_TIMEOUT_MS = 0
@@ -115,7 +115,7 @@ export async function run(options: RunOptions): Promise<number> {
     if (err instanceof Error && err.name === "AbortError") {
       return 130
     }
-    console.error(pc.red(`Error: ${err}`))
+    console.error(pc.red(`Error: ${serializeError(err)}`))
     return 1
   }
 }