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>

DIACHRON.md

@@ -88,8 +88,24 @@ There are two owners of files in a diachron project:
   `backend/diachron/`, and the top-level scripts (`cmd`, `develop`, `mgmt`,
   `sync.sh`, `check.sh`).
 
-Don't modify framework-owned files.  This separation keeps framework upgrades
-clean.
+Don't modify framework-owned files unless you need to.  This separation
+keeps framework upgrades clean.  If you do need to change framework files
+(especially early on, there are rough edges), you can extract your changes
+as a patch:
+
+```bash
+./diff-upstream.sh          # full diff against upstream
+./diff-upstream.sh --stat   # just list changed files
+```
+
+This diffs every file in `file-list` against the upstream ref recorded in
+`.diachron-version`.
+
+When you do change framework files, make each change in its own commit with
+a clear message explaining what the change is and why it's needed.  Mixing
+framework fixes with application work in a single commit makes it much
+harder to upstream later.  A clean history of discrete, well-explained
+framework commits is the easiest thing to turn into contributions.
 
 ## Getting started