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>

diff-upstream.sh

@@ -0,0 +1,116 @@
+#!/bin/bash
+
+# Generate a diff of framework files against the upstream version this
+# project is based on.  Useful for contributing changes back to diachron.
+#
+# Usage:
+#   ./diff-upstream.sh              # diff against .diachron-version
+#   ./diff-upstream.sh <ref>        # diff against a specific ref
+#   ./diff-upstream.sh --stat       # show changed files only
+
+set -eu
+set -o pipefail
+IFS=$'\n\t'
+
+DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+stat_only=false
+ref=""
+
+for arg in "$@"; do
+    case "$arg" in
+        --stat) stat_only=true ;;
+        *)      ref="$arg" ;;
+    esac
+done
+
+if [ -z "$ref" ]; then
+    if [ ! -f "$DIR/.diachron-version" ]; then
+        echo "Error: .diachron-version not found and no ref specified." >&2
+        echo "Usage: $0 [--stat] [<ref>]" >&2
+        exit 1
+    fi
+    ref=$(cat "$DIR/.diachron-version")
+fi
+
+cached_repo="$HOME/.cache/diachron/v1/repositories/diachron.git"
+
+if [ ! -d "$cached_repo" ]; then
+    echo "Error: cached repository not found at $cached_repo" >&2
+    echo "Run ./update-cached-repository.sh first." >&2
+    exit 1
+fi
+
+# Update cached repo
+"$DIR/update-cached-repository.sh"
+
+# Verify ref exists
+if ! git -C "$cached_repo" rev-parse --verify "$ref^{commit}" >/dev/null 2>&1; then
+    echo "Error: ref '$ref' not found in cached repository." >&2
+    exit 1
+fi
+
+# Read file-list
+files=()
+while IFS= read -r line; do
+    [[ "$line" =~ ^[[:space:]]*# ]] && continue
+    [[ -z "$line" ]] && continue
+    files+=("$line")
+done < "$DIR/file-list"
+
+# Check out upstream into a temp directory
+tmpdir=$(mktemp -d)
+cleanup() { rm -rf "$tmpdir"; }
+trap cleanup EXIT
+
+git clone --quiet "$cached_repo" "$tmpdir/upstream"
+git -C "$tmpdir/upstream" checkout --quiet "$ref"
+
+# Generate diff
+if $stat_only; then
+    diff -rq "$tmpdir/upstream" "$DIR" \
+        --no-dereference \
+        2>/dev/null \
+        | grep -v '^\.' \
+        || true
+
+    # Simpler: just list files that differ
+    for f in "${files[@]}"; do
+        # Skip directories
+        [ -d "$DIR/$f" ] && continue
+
+        upstream="$tmpdir/upstream/$f"
+        local="$DIR/$f"
+
+        if [ ! -f "$upstream" ] && [ -f "$local" ]; then
+            echo "added:    $f"
+        elif [ -f "$upstream" ] && [ ! -f "$local" ]; then
+            echo "removed:  $f"
+        elif [ -f "$upstream" ] && [ -f "$local" ]; then
+            if ! diff -q "$upstream" "$local" >/dev/null 2>&1; then
+                echo "modified: $f"
+            fi
+        fi
+    done
+else
+    for f in "${files[@]}"; do
+        [ -d "$DIR/$f" ] && continue
+
+        upstream="$tmpdir/upstream/$f"
+        local="$DIR/$f"
+
+        if [ ! -f "$upstream" ] && [ -f "$local" ]; then
+            diff -u /dev/null "$local" \
+                --label "a/$f" --label "b/$f" \
+                || true
+        elif [ -f "$upstream" ] && [ ! -f "$local" ]; then
+            diff -u "$upstream" /dev/null \
+                --label "a/$f" --label "b/$f" \
+                || true
+        elif [ -f "$upstream" ] && [ -f "$local" ]; then
+            diff -u "$upstream" "$local" \
+                --label "a/$f" --label "b/$f" \
+                || true
+        fi
+    done
+fi