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/events.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from "bun:test"
-import { createEventState, type EventState } from "./events"
+import { createEventState, serializeError, type EventState } from "./events"
 import type { RunContext, EventPayload } from "./types"
 
 const createMockContext = (sessionID: string = "test-session"): RunContext => ({
@@ -15,6 +15,63 @@ async function* toAsyncIterable<T>(items: T[]): AsyncIterable<T> {
   }
 }
 
+describe("serializeError", () => {
+  it("returns 'Unknown error' for null/undefined", () => {
+    // #given / #when / #then
+    expect(serializeError(null)).toBe("Unknown error")
+    expect(serializeError(undefined)).toBe("Unknown error")
+  })
+
+  it("returns message from Error instance", () => {
+    // #given
+    const error = new Error("Something went wrong")
+
+    // #when / #then
+    expect(serializeError(error)).toBe("Something went wrong")
+  })
+
+  it("returns string as-is", () => {
+    // #given / #when / #then
+    expect(serializeError("Direct error message")).toBe("Direct error message")
+  })
+
+  it("extracts message from plain object", () => {
+    // #given
+    const errorObj = { message: "Object error message", code: "ERR_001" }
+
+    // #when / #then
+    expect(serializeError(errorObj)).toBe("Object error message")
+  })
+
+  it("extracts message from nested error object", () => {
+    // #given
+    const errorObj = { error: { message: "Nested error message" } }
+
+    // #when / #then
+    expect(serializeError(errorObj)).toBe("Nested error message")
+  })
+
+  it("extracts message from data.message path", () => {
+    // #given
+    const errorObj = { data: { message: "Data error message" } }
+
+    // #when / #then
+    expect(serializeError(errorObj)).toBe("Data error message")
+  })
+
+  it("JSON stringifies object without message property", () => {
+    // #given
+    const errorObj = { code: "ERR_001", status: 500 }
+
+    // #when
+    const result = serializeError(errorObj)
+
+    // #then
+    expect(result).toContain("ERR_001")
+    expect(result).toContain("500")
+  })
+})
+
 describe("createEventState", () => {
   it("creates initial state with correct defaults", () => {
     // #given / #when