Reject duplicate host config sections during parsing

[ryanfowler]

Summary

• Reject duplicate host section names while parsing config files instead of letting later sections replace earlier ones
• Report the duplicate line and the first definition line in the error message
• Update config docs and AGENTS guidance to describe the new rule

Testing

• Added unit coverage for duplicate-host rejection in config parsing
• Added HTTP integration coverage that verifies a duplicate host section fails with a line-numbered config error