feat: auto-restart headless mode on crash with exponential backoff (#886) (#897)

When the headless child process crashes or errors out, auto-restart
with exponential backoff (5s, 10s, 15s... up to 30s) instead of
exiting immediately. This enables overnight 'fire and forget' runs.

- --max-restarts N (default 3, 0 to disable): controls restart budget
- Only restarts on crashes (exit code !== 0), not on success or blocked
- SIGINT/SIGTERM bypasses restart (user intent to stop)
- Restart count shown in summary output
- Backoff prevents rapid crash loops from burning API credits

The inner loop function (runHeadlessOnce) returns exit status instead
of calling process.exit, letting the outer loop decide whether to
restart or terminate.

This is the first step toward the 'absolute autonomy' goal described
in #886 — process-level resilience for long-running sessions.

src/headless.ts

@@ -33,6 +33,7 @@ export interface HeadlessOptions {
   contextText?: string   // inline text
   auto?: boolean         // chain into auto-mode after milestone creation
   verbose?: boolean      // show tool calls in output
+  maxRestarts?: number   // auto-restart on crash (default 3, 0 to disable)
 }
 
 interface ExtensionUIRequest {
@@ -92,6 +93,12 @@ export function parseHeadlessArgs(argv: string[]): HeadlessOptions {
         options.auto = true
       } else if (arg === '--verbose') {
         options.verbose = true
+      } else if (arg === '--max-restarts' && i + 1 < args.length) {
+        options.maxRestarts = parseInt(args[++i], 10)
+        if (Number.isNaN(options.maxRestarts) || options.maxRestarts < 0) {
+          process.stderr.write('[headless] Error: --max-restarts must be a non-negative integer\n')
+          process.exit(1)
+        }
       }
     } else if (!positionalStarted) {
       positionalStarted = true
@@ -279,6 +286,37 @@ function bootstrapGsdProject(basePath: string): void {
 }
 
 export async function runHeadless(options: HeadlessOptions): Promise<void> {
+  const maxRestarts = options.maxRestarts ?? 3
+  let restartCount = 0
+
+  while (true) {
+    const result = await runHeadlessOnce(options, restartCount)
+
+    // Success or blocked — exit normally
+    if (result.exitCode === 0 || result.exitCode === 2) {
+      process.exit(result.exitCode)
+    }
+
+    // Crash/error — check if we should restart
+    if (restartCount >= maxRestarts) {
+      process.stderr.write(`[headless] Max restarts (${maxRestarts}) reached. Exiting.\n`)
+      process.exit(result.exitCode)
+    }
+
+    // Don't restart if SIGINT/SIGTERM was received
+    if (result.interrupted) {
+      process.exit(result.exitCode)
+    }
+
+    restartCount++
+    const backoffMs = Math.min(5000 * restartCount, 30_000)
+    process.stderr.write(`[headless] Restarting in ${(backoffMs / 1000).toFixed(0)}s (attempt ${restartCount}/${maxRestarts})...\n`)
+    await new Promise(resolve => setTimeout(resolve, backoffMs))
+  }
+}
+
+async function runHeadlessOnce(options: HeadlessOptions, restartCount: number): Promise<{ exitCode: number; interrupted: boolean }> {
+  let interrupted = false
   const startTime = Date.now()
   const isNewMilestone = options.command === 'new-milestone'
 
@@ -452,6 +490,7 @@ export async function runHeadless(options: HeadlessOptions): Promise<void> {
   // Signal handling
   const signalHandler = () => {
     process.stderr.write('\n[headless] Interrupted, stopping child process...\n')
+    interrupted = true
     exitCode = 1
     client.stop().finally(() => {
       clearTimeout(timeoutTimer)
@@ -553,6 +592,9 @@ export async function runHeadless(options: HeadlessOptions): Promise<void> {
   process.stderr.write(`[headless] Status: ${status}\n`)
   process.stderr.write(`[headless] Duration: ${duration}s\n`)
   process.stderr.write(`[headless] Events: ${totalEvents} total, ${toolCallCount} tool calls\n`)
+  if (restartCount > 0) {
+    process.stderr.write(`[headless] Restarts: ${restartCount}\n`)
+  }
 
   // On failure, print last 5 events for diagnostics
   if (exitCode !== 0) {
@@ -565,5 +607,5 @@ export async function runHeadless(options: HeadlessOptions): Promise<void> {
     }
   }
 
-  process.exit(exitCode)
+  return { exitCode, interrupted }
 }