Delete stale root-level Dockerfiles

Dockerfile, Dockerfile.claude and Dockerfile.win2022 at the repo root
have been dead code since commit 108f45f (Feb 6) moved Dockerfile
generation to eng/docker/. DockerBuild.ps1 resolves Dockerfiles from
$EngPath/docker exclusively, and ContainerRequirements.WriteDockerfileCore
writes to that same directory, so the root copies were never read or
rewritten — they just drifted (the Claude CLI version was still pinned
to 2.1.34 from two months ago) and confused anyone reading them.

Also fix ClaudeCodeHelper.cs's merge-conflict guide, which still listed
the old root paths as auto-generated files.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: PostSharpAgent

Dockerfile

@@ -446,8 +446,10 @@ This is a NON-INTERACTIVE session - you must complete the task without asking qu
                 (preferably the current/HEAD version). They will be regenerated when the PR build runs.
                 - Build.ps1
                 - DockerBuild.ps1
-                - Dockerfile
-                - Dockerfile.claude
+                - eng/docker/Dockerfile
+                - eng/docker/Dockerfile.claude
+                - eng/docker/Dockerfile.win2022
+                - eng/docker/Dockerfile.claude.win2022
                 - .teamcity/settings.kts
                 - .teamcity/pom.xml
                 - eng/Versions.*.g.props