Refactor env int parsing and clarify logger sink intent

[Copilot]

The semantic clustering review found three refactor opportunities: ambiguous logger sink naming, duplicated int env parsing logic, and a generic JSON extraction helper hidden in DIFC logging code. This PR takes the low-risk path: consolidate parsing behavior and improve discoverability/intent via docs and comments without API churn.

• Env parsing consolidation

  • Added envutil.GetEnvIntRaw(envKey) (int, bool, error) to centralize raw integer env parsing (present/not-present/parse-error semantics).
  • Refactored config.parseAndValidateIntEnv to delegate parsing to GetEnvIntRaw, keeping domain validation in config.
  • Updated GetEnvInt to reuse GetEnvIntRaw internally, removing duplicate parse logic.

• Logger sink clarity (no rename)

  • Clarified godoc on LogInfo/LogWarn/LogError/LogDebug that unsuffixed functions target the unified file sink (mcp-gateway.log).
  • Added matching guidance in AGENTS.md to direct destination-specific usage toward Log*ToMarkdown / Log*ToServer.

• DIFC helper discoverability

  • Added a targeted note on getFilteredItemStringField in internal/server/difc_log.go indicating it is intentionally generic and a candidate for future extraction if reuse grows.

• Focused coverage update

  • Added tests for the new GetEnvIntRaw behavior in internal/envutil/envutil_test.go.

// config now delegates parse to envutil, then applies domain validation.
value, present, err := envutil.GetEnvIntRaw(envKey)
if !present {
	return 0, false, nil
}
if err != nil {
	return 0, false, fmt.Errorf("invalid %s value", envKey)
}
if validationErr := validate(value); validationErr != nil {
	return 0, false, fmt.Errorf("%s", validationErr.Message)
}

[Copilot]

Pull request overview

This PR refactors integer environment-variable parsing to centralize raw parsing semantics in envutil, and clarifies the intent/destination of logger sinks via improved godoc and documentation notes.

Changes:

• Added envutil.GetEnvIntRaw(envKey) (int, bool, error) and refactored GetEnvInt + config.parseAndValidateIntEnv to reuse it.
• Clarified logger godoc/agent guidance to distinguish unsuffixed Log* (unified file sink) from destination-specific Log*ToMarkdown / Log*ToServer.
• Added tests covering GetEnvIntRaw behavior and a discoverability note for a generic DIFC JSON extraction helper.

Show a summary per file

File	Description
internal/server/difc_log.go	Adds a note clarifying the generic nature of getFilteredItemStringField for future extraction.
internal/logger/file_logger.go	Updates exported Log* godoc to clarify it targets the unified file sink.
internal/envutil/envutil.go	Introduces GetEnvIntRaw and refactors GetEnvInt to delegate parsing.
internal/envutil/envutil_test.go	Adds unit tests for the new GetEnvIntRaw helper.
internal/config/config_env.go	Refactors int env parsing/validation to delegate parsing to envutil.GetEnvIntRaw.
AGENTS.md	Adds usage guidance distinguishing unified vs destination-specific logging helpers.

Copilot's findings

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• Files reviewed: 6/6 changed files
• Comments generated: 2