fix(exec): detect auth failures in stderr and abort early (#285)

[vybe]

Summary

• Add real-time auth failure detection in agent server stderr scanning
• Kill Claude Code process immediately when expired token patterns detected
• Return HTTP 503 so backend can classify as error_code=AUTH
• Add fallback heuristics: auth pattern in stderr, zero tokens processed

Problem

When subscription tokens (CLAUDE_CODE_OAUTH_TOKEN) expire, Claude Code sometimes hangs instead of failing fast. This wastes execution slots for up to an hour before the watchdog recovers them. Issue #285 reports some executions fail in ~3 minutes (correct) while others hang for the full timeout (bug).

Solution

Detect auth failure patterns in stderr during execution (not after) and abort immediately:

1. _is_auth_failure_message() pattern matcher scans for "token expired", "unauthorized", etc.
2. read_stderr() thread checks each line and sets abort event if pattern found
3. Process is killed immediately, HTTP 503 returned
4. Backend detects 503, sets error_code=TaskExecutionErrorCode.AUTH

Test Plan

• Unit tests for pattern matcher: pytest tests/test_auth_failure_detection.py -v
• Manual: Configure agent with expired token, verify fails in seconds not hours
• Verify watchdog still recovers executions that slip through

Closes #285

🤖 Generated with Claude Code