feat: make Supervisor logfile path configurable via environment variable

Added SUPERVISOR_LOGFILE environment variable to configure the path to
the Supervisor main logfile for reading via XML-RPC.

Changes:
- Added SUPERVISOR_LOGFILE to .env.example with default path
- Removed getLogfilePath() method and cachedLogfilePath field from SupervisorClient
- Updated readLog() to use environment variable with fallback chain:
  1. Explicitly provided logfilePath parameter
  2. SUPERVISOR_LOGFILE environment variable
  3. Default: /var/log/supervisor/supervisord.log
- Added debug logging to readLog() for troubleshooting

This allows users to configure the correct logfile path for their
Supervisor installation when using the custom 3-parameter XML-RPC
readLog implementation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.env.example

@@ -11,6 +11,10 @@ SUPERVISOR_PORT=9001
 # SUPERVISOR_USERNAME=user
 # SUPERVISOR_PASSWORD=pass
 
+# Optional: Path to supervisord main logfile (for reading via XML-RPC)
+# Default: /var/log/supervisor/supervisord.log
+# SUPERVISOR_LOGFILE=/var/log/supervisor/supervisord.log
+
 # Logging Configuration
 # Log level: debug, info, warn, error (default: info in prod, debug in dev)
 LOG_LEVEL=info