Build dmr.exe on Windows in build-dmr target

[ericcurtin]

Summary

• The build-dmr Makefile target always built a binary named "dmr",
  which is unusable on Windows because the OS requires a .exe suffix
  to execute it directly
• Detect Windows via the built-in $(OS) variable and use a
  DMR_EXE variable for the output filename, so the target produces
  dmr.exe on Windows and dmr everywhere else. This avoids
  shelling out to go env GOEXE on every invocation, which adds
  overhead and can fail if go is not on the PATH
• Reuse DMR_EXE in the clean target instead of hardcoding both
  dmr and dmr.exe so cleanup stays in sync with the build output
• Update the README to match

Test plan

• make validate-all (go mod tidy, lint, tests w/ race detection, shellcheck) passes
• make build-dmr and make clean verified locally on macOS

[sourcery-ai]

Hey - I've left some high level feedback:

• Consider reusing the DMR_EXE variable in the clean target instead of hardcoding both dmr and dmr.exe so the cleanup behavior automatically stays in sync with GOEXE-based naming.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider reusing the DMR_EXE variable in the clean target instead of hardcoding both dmr and dmr.exe so the cleanup behavior automatically stays in sync with GOEXE-based naming.

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[gemini-code-assist]

Code Review

This pull request adds support for building the dmr binary with a .exe extension on Windows by updating the Makefile and README. Feedback suggests replacing the shell call to 'go env GOEXE' with a check on the built-in '$(OS)' variable to avoid unnecessary overhead and potential build failures when Go is not in the PATH.

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