[log] Add debug logging to internal/server/server.go

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Summary

This PR adds debug logging to internal/server/server.go to enhance troubleshooting and visibility into server operations.

Changes

• Added debug logger with namespace server:server following the pkg:filename convention
• Added 7 logging statements to track key operations:
  1. Server creation with mode (unified vs routed)
  2. Route setup for different modes
  3. Handler registration for routed/unified endpoints
  4. Server selection in unified mode
  5. Path parsing in routed mode
  6. Request proxying decisions (including sys server routing)
  7. Server startup with listening address

Benefits

• Better visibility into server mode selection and initialization
• Debug tracking of request routing decisions
• Easier troubleshooting of proxy target selection
• Consistent logging infrastructure following project conventions

Testing

Changes follow the project's logging guidelines from AGENTS.md:

• Logger uses pkg:filename naming convention
• No side effects in logger arguments
• Logs meaningful context for debugging
• Complements existing operational logging

Enable debug output with:

DEBUG=server:server ./awmg --config config.toml
# or for all server logs
DEBUG=server:* ./awmg --config config.toml

Checklist

• Single file modified (focused PR)
• Logger naming follows pkg:filename convention
• Logger arguments have no side effects
• Logging messages are meaningful and helpful
• No duplicate logging with existing logs
• Import statements properly formatted

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