Bug: verifyreadmes does not exclude hidden dot-directories on Windows

[RedZapdos123]

Description:

On Windows, internal/tools/verifyreadmes does not exclude hidden dot-directories like .hidden, even though the current exclusion logic skips them on slash-separated platforms.

Steps to Reproduce:

1. Check out main at b05b3d3bd.

2. From PowerShell in the repository root, run:

   $root = Join-Path $env:TEMP 'otel-verifyreadmes-hidden'
   Remove-Item -Recurse -Force $root -ErrorAction SilentlyContinue
   
   $mod = Join-Path $root '.hidden\mod'
   New-Item -ItemType Directory -Force -Path $mod | Out-Null
   Set-Content -Path (Join-Path $mod 'go.mod') -Value "module example.com/hidden`ngo 1.25.0`n"
   
   go run ./internal/tools/verifyreadmes $root

Actual Behavior:

The tool exits non-zero and reports a missing README.md inside the hidden directory instead of skipping it.

Example output:

Verifying READMEs in C:\Users\<user>\AppData\Local\Temp\otel-verifyreadmes-hidden
Some readme files couldn't be found.
couldn't find README.md for "C:\\Users\\<user>\\AppData\\Local\\Temp\\otel-verifyreadmes-hidden\\.hidden\\mod"

Expected Behavior:

Hidden dot-directories should be excluded consistently on Windows as well, so .hidden\mod\go.mod should be skipped and the command should succeed.

Notes:

This appears to come from excludedDirs containing "/." while verifyReadme checks strings.Contains(path, dir) against OS-native paths. On Windows, the walked path uses \, so paths like C:\...\\.hidden\\mod\\go.mod never match the "/." exclusion.

This affects the internal verifyreadmes tool used by make verify-readmes and make precommit.

[RedZapdos123]

Opened a PR to resolve the issue.