feat: improve spawn error handling and reporting across full stack (#249)

* feat: improve spawn error handling and reporting across full stack

- Return error result instead of throwing in apiMachine spawn handler
- Add lastSpawnError field to RunnerState for persistent error tracking
- Add error awaiter system for early process exit/error detection before webhook
- Build detailed webhook failure messages with exit code, signal, and stderr tail
- Report spawn outcomes to hub via runner state updates
- Handle more spawn result types in rpcGateway with better error messages
- Display runner last spawn error in web UI (NewSession & SpawnSession)
- Extract shared formatRunnerSpawnError utility to avoid duplication

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(cli): narrow spawnResult type check to fix TS2339 error

Use `type === 'error'` instead of `type !== 'success'` to properly
narrow the discriminated union, allowing TypeScript to infer errorMessage.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

web/src/components/NewSession/index.tsx

@@ -21,6 +21,7 @@ import {
 } from './preferences'
 import { SessionTypeSelector } from './SessionTypeSelector'
 import { YoloToggle } from './YoloToggle'
+import { formatRunnerSpawnError } from '../../utils/formatRunnerSpawnError'
 
 export function NewSession(props: {
     api: ApiClient
@@ -82,6 +83,15 @@ export function NewSession(props: {
         }
     }, [props.machines, machineId, getLastUsedMachineId, getRecentPaths])
 
+    const selectedMachine = useMemo(
+        () => (machineId ? props.machines.find((machine) => machine.id === machineId) ?? null : null),
+        [machineId, props.machines]
+    )
+    const runnerSpawnError = useMemo(
+        () => formatRunnerSpawnError(selectedMachine),
+        [selectedMachine]
+    )
+
     const recentPaths = useMemo(
         () => getRecentPaths(machineId),
         [getRecentPaths, machineId]
@@ -247,6 +257,11 @@ export function NewSession(props: {
                 isDisabled={isFormDisabled}
                 onChange={handleMachineChange}
             />
+            {runnerSpawnError ? (
+                <div className="px-3 py-2 text-xs text-red-600">
+                    Runner last spawn error: {runnerSpawnError}
+                </div>
+            ) : null}
             <DirectorySection
                 directory={directory}
                 suggestions={suggestions}

web/src/components/SpawnSession.tsx

@@ -5,6 +5,7 @@ import { Button } from '@/components/ui/button'
 import { Card, CardContent, CardDescription, CardHeader, CardTitle } from '@/components/ui/card'
 import { usePlatform } from '@/hooks/usePlatform'
 import { useSpawnSession } from '@/hooks/mutations/useSpawnSession'
+import { formatRunnerSpawnError } from '@/utils/formatRunnerSpawnError'
 import { useTranslation } from '@/lib/use-translation'
 
 type SessionType = 'simple' | 'worktree'
@@ -32,6 +33,10 @@ export function SpawnSession(props: {
     const { spawnSession, isPending, error: spawnError } = useSpawnSession(props.api)
 
     const machineTitle = useMemo(() => getMachineTitle(props.machine), [props.machine])
+    const runnerSpawnError = useMemo(
+        () => formatRunnerSpawnError(props.machine),
+        [props.machine?.runnerState?.lastSpawnError]
+    )
 
     async function spawn() {
         const trimmed = directory.trim()
@@ -144,6 +149,12 @@ export function SpawnSession(props: {
                             </div>
                         </div>
 
+                        {runnerSpawnError ? (
+                            <div className="text-xs text-red-600">
+                                Runner last spawn error: {runnerSpawnError}
+                            </div>
+                        ) : null}
+
                         {(error ?? spawnError) ? (
                             <div className="text-sm text-red-600">
                                 {error ?? spawnError}