fix(stack): fall back to USERNAME for branch prefix

Tien Rayrequested to merge
ray.tien0907/cli:fix/8301-windows-username into main

Summary

  • preserve the existing configured branch_prefix and USER precedence
  • fall back to USERNAME before glab-stack when no prefix is configured
  • cover the complete precedence matrix and update generated help and bundled stack guidance

Testing

  • go test -count=1 ./internal/commands/stack/infer -run '^TestCreateShaBranch$' -v
  • go test -count=1 ./internal/commands/stack/infer ./internal/commands/stack/save ./internal/commands/stack/stackutils ./internal/commands/config ./internal/commands/skills/bundled
  • focused internal/config schema tests
  • make build
  • make lint (0 issues)
  • generated docs and Go code checks
  • pre-push changed tests: 3,483 tests run; the only failures were the two pre-existing linked-worktree path assertions, Test_LocalConfigDir and Test_LocalConfigFile, reproduced unchanged before this patch

Closes #8301 (closed)

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