Add diff-upstream.sh and document framework change workflow
New script extracts a diff of framework files against the upstream ref in .diachron-version. Both DIACHRON.md and diachron/AGENTS.md now explain how to use it and recommend keeping framework changes in discrete, well-described commits to ease upstreaming. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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