Pass a custom changelog to manage-release by file, not by cross-job input #649

Problem

The built-in changelog now passes to the manage-release action by file, which avoids the single-environment-variable size limit (see #640). A user-supplied custom changelog, however, still crosses a job boundary via a workflow output and input, so a pathologically large custom changelog could hit workflow output/input size limits.

Proposed

Unify the custom-changelog path onto the same file/artifact-based passing the built-in path uses, so no changelog content transits a workflow output or environment variable regardless of source.

Severity

Low. Only affects a user-supplied custom changelog large enough to exceed the limits; not a regression. Tracked as the R1 residual from the v1 base audit.