ci: guide SEP change PRs to engage authors

[marcelosalloum]

What

• Add a workflow that converts PRs modifying existing SEPs to draft (without locking) and posts a guidance comment.
• The comment links each modified SEP's existing Discussion: thread and @-mentions its original authors, parsed from the preamble Author: line (only valid GitHub handles; plain names, emails, and org authors are skipped).

Why

• Per the SEP Contribution Process, changes to an existing SEP should start as a discussion and be advocated with the SEP's author(s) — this automates that guidance.
• Draft status reserves "ready for review" to mean the change has been discussed and a maintainer should review and merge.

When it triggers

• pull_request_target on opened/reopened for PRs that modify existing ecosystem/sep-*.md files.
• Defers to the New SEP Proposal workflow when the PR adds a new SEP; posts at most once per PR (idempotency marker).

Example comment

For a PR by @somecontributor modifying sep-0038.md:

👋 Thanks for your contribution!

This PR proposes changes to the following existing SEP(s):

• ecosystem/sep-0038.md — author(s): @jakeurban, @leighmcculloch — existing discussion

Per the SEP Contribution Process, changes to an existing SEP should start as a discussion, and should be advocated with the SEP's author(s) — the author maintains the proposal, so engaging them is an important part of getting a change adopted. If you haven't already, please raise your proposed change on the SEP's existing discussion linked above, or start one on the GitHub discussion forum.

I have marked this PR as a draft for now. Once the change has been discussed and is ready for a maintainer to review and merge, mark it as ready for review.

Thanks for helping improve the Stellar ecosystem! 🚀

[Copilot]

Pull request overview

Adds automation to guide pull requests that modify existing SEPs through the contribution process.

Changes:

• Detects modified SEP files and extracts authors/discussion links.
• Converts qualifying PRs to drafts.
• Posts idempotent contribution guidance.

[leighmcculloch]

Could we add a comment to the top highlighting it is critical the job does not check out pr data?