fix: fall back to stderr (not stdout) when log-dir is unwritable

[Copilot]

When awmg cannot open log files in --log-dir, it was falling back to os.Stdout — corrupting the stdout JSON channel that start_mcp_gateway.cjs parses to get the gateway config. The caller sees a SyntaxError with no actionable error from awmg, since log lines like [2026-...] [INFO] [startup] are valid-looking JSON array starts.

Changes

• internal/logger/file_logger.go — handleFileLoggerError: os.Stdout → os.Stderr; GetWriter() fallback return: os.Stdout → os.Stderr; fallback warning message updated accordingly
• internal/logger/common.go — Update doc comment describing the FileLogger fallback strategy
• internal/logger/file_logger_test.go — Update test subcase and assertion to expect os.Stderr

// Before
func handleFileLoggerError(err error, logDir, fileName string) (*FileLogger, error) {
    logFallbackWarnings(err, "Failed to initialize log file", "Falling back to stdout for logging")
    fl := &FileLogger{..., logger: log.New(os.Stdout, "", 0)}
    return fl, nil
}

// After
func handleFileLoggerError(err error, logDir, fileName string) (*FileLogger, error) {
    logFallbackWarnings(err, "Failed to initialize log file", "Falling back to stderr for logging")
    fl := &FileLogger{..., logger: log.New(os.Stderr, "", 0)}
    return fl, nil
}

[Copilot]

Pull request overview

Fixes a bug where falling back to os.Stdout for logging when the log directory was unwritable would corrupt the stdout JSON channel parsed by start_mcp_gateway.cjs. The fix switches the fallback to os.Stderr across the FileLogger implementation, its GetWriter method, and updates the related doc comments and test.

Changes:

• Switch FileLogger fallback writer from os.Stdout to os.Stderr in handleFileLoggerError and GetWriter.
• Update documentation comments (common.go, file_logger.go) to reflect the new fallback target and rationale.
• Update test subcase name and assertion in file_logger_test.go to expect os.Stderr.

Show a summary per file

File	Description
internal/logger/file_logger.go	Switch fallback writer to stderr; update doc comments and warning string.
internal/logger/common.go	Update FileLogger fallback strategy doc comment to mention stderr.
internal/logger/file_logger_test.go	Update test subcase name and assertion to expect os.Stderr.

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• Files reviewed: 3/3 changed files
• Comments generated: 0