Clarify copyright header policy for fork-added and upstream files

[bernalde]

While preparing Pyomo integration work in the SECQUOIA fork, we noticed that copyright headers can become ambiguous when a downstream fork adds new files and modifies existing upstream files.

The concern is that upstream LyoPRONTO does not belong to SECQUOIA, so fork-side PRs should not accidentally imply SECQUOIA ownership over upstream-authored files. Our current working convention in the SECQUOIA fork is:

• Leave existing upstream copyright headers intact for files that already existed upstream.
• Use Copyright (C) 2026, SECQUOIA only for files newly added by SECQUOIA.
• Avoid rewriting untouched upstream files just to normalize headers.

Questions for upstream:

1. What copyright/header convention should contributors follow for files that already existed upstream but are modified by downstream contributors?
2. Should new contributor or organization notices be added as extra header lines, recorded in a central AUTHORS/NOTICE file, or omitted from per-file headers?
3. Should upstream document a repository-wide policy so future PRs apply the same convention consistently?

This came up while working on a stacked set of PRs in the SECQUOIA fork for Pyomo integration. Depending on the preferred policy, several PRs in that stack may need to be updated to keep copyright notices consistent before they are proposed upstream.