fix: hide backend subprocess windows on Windows

[Priyanchew]

Summary

• add a shared backend process helper that hides non-interactive child process windows on Windows
• route production git, gh, workspace, SCM, legacy import, and shell-command subprocesses through the helper
• keep existing non-Windows behavior unchanged

Verification

• Manual packaged Windows install/run verified: git/gh subprocess flashes are gone
• go test ./internal/process ./internal/cli ./internal/adapters/scm/github ./internal/service/project ./internal/observe/scm ./internal/session_manager

[harshitsinghbhandari]

lgtm