fix(stack): do not leak a file handle in AddStackBaseBranch

What does this MR do and why?

AddStackBaseBranch calls os.Create and discards the returned file without closing it, then writes the same path again with os.WriteFile:

_, err = os.Create(filename)
if err != nil {
	return err
}

data := []byte(branch)

err = os.WriteFile(filename, data, 0o644)

The os.Create is redundant, since os.WriteFile creates the file itself, and the discarded handle stays open for the life of the process. On Windows that open handle blocks the file from being removed, which the existing test already trips over:

--- FAIL: Test_AddStackBaseBranch/successfully_add_custom_branch
    TempDir RemoveAll cleanup: unlinkat ...\stacked\custom-stack\BASE_BRANCH:
    The process cannot access the file because it is being used by another process.

Removing the os.Create fixes both the leak and that failure.

Test coverage

No new test: the existing Test_AddStackBaseBranch is already the regression case. It fails on main on Windows with the cleanup error above and passes with this change, so adding another test would only duplicate it.

go test ./internal/git/ is green.

Related issues

Closes #8436 (closed)