feat: backport auth-validation preflight + email_login credentials

Backport upstream Shannon PR #335:
- Add credential validation activity that drives a real browser login
  before the full pipeline, catching bad credentials early
- New email_login credentials type for magic-link and email-OTP flows
- Make credentials.password optional for passwordless flows
- Playwright stealth config (chrome.runtime, plugin simulation, UA)
- Centralize prompt directory resolution into resolvePromptDir helper
- New AUTH_LOGIN_FAILED error code with non-retryable classification
- Remove dangerous-pattern validation on credentials.password
- Pipeline-testing stub for auth validation (returns success)
- Auth validation timeout of 10 minutes for browser-based login
- .playwright directory workspace overlay for CLI/Docker

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/worker/configs/config-schema.json

@@ -39,9 +39,33 @@
               "type": "string",
               "pattern": "^[A-Za-z2-7]+=*$",
               "description": "TOTP secret for two-factor authentication (Base32 encoded, case insensitive)"
+            },
+            "email_login": {
+              "type": "object",
+              "description": "Email-based login credentials for magic-link and email-OTP flows",
+              "properties": {
+                "address": {
+                  "type": "string",
+                  "format": "email",
+                  "description": "Email address for authentication"
+                },
+                "password": {
+                  "type": "string",
+                  "minLength": 1,
+                  "maxLength": 255,
+                  "description": "Password for the email account"
+                },
+                "totp_secret": {
+                  "type": "string",
+                  "pattern": "^[A-Za-z2-7]+=*$",
+                  "description": "TOTP secret for email 2FA (Base32 encoded)"
+                }
+              },
+              "required": ["address", "password"],
+              "additionalProperties": false
             }
           },
-          "required": ["username", "password"],
+          "required": ["username"],
           "additionalProperties": false
         },
         "login_flow": {