fix(error): preserve Error.cause chain in non-debug output

Addresses Cursor bugbot feedback on PR #1238. The previous code walked
the cause chain only when `showStack`/verbose was set, so non-debug users
lost the wrapped-error diagnostic context that `messageWithCauses` used
to concatenate automatically.

- Always fold up to 5 `Error.cause` messages into `message` for generic
  Error instances, so the outer message reads like
  `outer: middle: root` — matching `pony-cause`'s behavior.
- The verbose/debug path still renders the richer `Caused by [N]:`
  formatted body with stack traces.
- Add regression tests covering both the cause-chain preservation and
  the 5-level truncation cap.

Author: jdalton

packages/cli/src/utils/error/display.mts

@@ -74,7 +74,23 @@ export function formatErrorForDisplay(
     body = error.body
   } else if (error instanceof Error) {
     title = opts.title || 'Unexpected error'
+    // Concatenate the cause chain into `message` (what non-debug users see)
+    // so diagnostic context from wrapped errors isn't silently dropped.
+    // `showStack` adds a richer formatted body with stack traces below.
     message = error.message
+    const plainCauses: string[] = []
+    let walk: unknown = error.cause
+    let walkDepth = 1
+    while (walk && walkDepth <= 5) {
+      plainCauses.push(
+        walk instanceof Error ? walk.message : String(walk),
+      )
+      walk = walk instanceof Error ? walk.cause : undefined
+      walkDepth++
+    }
+    if (plainCauses.length) {
+      message = `${message}: ${plainCauses.join(': ')}`
+    }
 
     if (showStack && error.stack) {
       // Format stack trace with proper indentation.

packages/cli/test/unit/utils/error/display.test.mts

@@ -150,6 +150,38 @@ describe('error/display', () => {
       expect(result.message).toBe('Something went wrong')
     })
 
+    it('preserves Error.cause chain in message without debug mode', () => {
+      // Regression: formatErrorForDisplay used to only surface causes
+      // under showStack/verbose, so non-debug users lost the most useful
+      // diagnostic context. See PR #1238.
+      const inner = new Error('root DNS failure')
+      const middle = new Error('network call failed', { cause: inner })
+      const outer = new Error('API request failed', { cause: middle })
+
+      const result = formatErrorForDisplay(outer)
+
+      expect(result.message).toContain('API request failed')
+      expect(result.message).toContain('network call failed')
+      expect(result.message).toContain('root DNS failure')
+    })
+
+    it('stops walking causes at depth 5 to avoid runaway chains', () => {
+      // Build inside-out so the outer Error sits at index 10 and chains
+      // down through level-9, level-8, ..., level-0.
+      let e: Error | undefined
+      for (let i = 0; i <= 10; i++) {
+        e = new Error(`level-${i}`, e ? { cause: e } : undefined)
+      }
+
+      const result = formatErrorForDisplay(e!)
+
+      // Top message + 5 causes should appear.
+      expect(result.message).toContain('level-10')
+      expect(result.message).toContain('level-5')
+      // Anything beyond depth 5 should have been truncated.
+      expect(result.message).not.toContain('level-4')
+    })
+
     it('uses custom title when provided', () => {
       const error = new Error('Something went wrong')