L2 Builder Uni: Trim redundant fields from rsyslog JSON envelope

[KA-ROM]

Summary

Removes 3 redundant fields from rsyslog JSON envelope templates (sysloghost, severity, facility). All live mainnet builder logs carry these duplications:

• Hostname (2x): jsonPayload.sysloghost duplicates labels.compute.googleapis.com/resource_name
• Severity (2x): jsonPayload.severity always reports info regardless of actual app log level. Misleading and duplicated by jsonPayload.message.level
• Facility: always "daemon"

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AI generated:

  ┌─────────────┬───────────────┬──────────────┬─────────┐
  │   Source    │ Avg line size │ Removed/line │ Savings │
  ├─────────────┼───────────────┼──────────────┼─────────┤
  │ bproxy      │ 718 bytes     │ 90 bytes     │ 12.5%   │
  ├─────────────┼───────────────┼──────────────┼─────────┤
  │ op-rbuilder │ 462 bytes     │ 90 bytes     │ 19.5%   │
  └─────────────┴───────────────┴──────────────┴─────────┘

 ~90 bytes removed is constant. Percentage depends on how fat the line is: short op-rbuilder lines 
 save more proportionally, longer bproxy lines save less.