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/utils/formatRunnerSpawnError.ts

@@ -0,0 +1,15 @@
+import type { Machine } from '../types/api'
+
+export function formatRunnerSpawnError(machine: Machine | null): string | null {
+    const lastSpawnError = machine?.runnerState?.lastSpawnError
+    if (!lastSpawnError?.message) {
+        return null
+    }
+
+    const at = typeof lastSpawnError.at === 'number'
+        ? new Date(lastSpawnError.at).toLocaleString()
+        : null
+    return at
+        ? `${lastSpawnError.message} (${at})`
+        : lastSpawnError.message
+}