[log] Add debug logging to config_core.go

[github-actions]

Adds 5 meaningful debug logging calls to internal/config/config_core.go using the existing logConfig logger ("config:config").

Changes

applyGatewayDefaults — logs resolved defaults after all zero-value fields are filled in:

Applied gateway defaults: port=3000, startupTimeout=30, toolTimeout=60, keepaliveInterval=1500

LoadFromFile — adds intermediate progress logs between the existing entry/exit log calls:

1. Before unknown-field check: Checking N undecoded TOML keys against allowed fields
2. Before stdio containerization validation: Validating stdio server containerization requirements for N servers
3. Before auth validation loop: Validating auth configuration for N servers
4. When opentelemetry section is present: opentelemetry section found: merging into tracing config, endpoint=...

Why

LoadFromFile already logs at entry and exit but its many intermediate validation steps were invisible under debug logging. These additions make it easy to pinpoint which validation step fails or how far initialization progressed when troubleshooting startup issues with DEBUG=config:*.

Validation

• go build ./... — passes ✅
• go vet ./... — passes ✅
• go test ./internal/... — all packages pass; only TestFetchAndFixSchema_NetworkError fails, which is a pre-existing network-dependent test failure unrelated to these changes ✅

Warning

⚠️ Firewall blocked 1 domain

The following domain was blocked by the firewall during workflow execution:

• invalidhostthatdoesnotexist12345.com

To allow these domains, add them to the network.allowed list in your workflow frontmatter:

network:
  allowed:
    - defaults
    - "invalidhostthatdoesnotexist12345.com"

See Network Configuration for more information.

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