Add social share card (og:image) + summary_large_image

Brand-accurate 1200x630 OG card (paper bg, navy serif headline, amber accent,
navy footer band). Wires og:image/twitter:image so shared links render a card
on LinkedIn/iMessage/Slack instead of a blank preview. SVG source kept alongside.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

assets/og-card.svg

@@ -0,0 +1,21 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1200 630" width="1200" height="630">
+  <!-- Open Communications — social share card (1200x630). Rasterize to og-card.png to enable. -->
+  <rect width="1200" height="630" fill="#f5f2ec"/>
+  <rect x="0" y="0" width="1200" height="6" fill="#e89b1c"/>
+
+  <!-- wordmark -->
+  <text x="80" y="118" font-family="'DM Sans', system-ui, sans-serif" font-size="29" letter-spacing="7" font-weight="600" fill="#0f1729">OPEN COMMUNICATIONS</text>
+
+  <!-- eyebrow -->
+  <text x="82" y="292" font-family="'DM Sans', system-ui, sans-serif" font-size="22" letter-spacing="4" fill="#7a6f5d">THE HUMAN SIDE OF CHANGE</text>
+
+  <!-- headline -->
+  <text x="78" y="378" font-family="'DM Serif Display', Georgia, 'Times New Roman', serif" font-size="86" fill="#0f1729">The systems are the</text>
+  <text x="78" y="470" font-family="'DM Serif Display', Georgia, 'Times New Roman', serif" font-size="86" font-style="italic" fill="#e89b1c">easy part.</text>
+
+  <!-- footer band -->
+  <rect x="0" y="558" width="1200" height="72" fill="#0f1729"/>
+  <circle cx="94" cy="595" r="7" fill="#e89b1c"/>
+  <text x="116" y="603" font-family="'DM Sans', system-ui, sans-serif" font-size="22" letter-spacing="2" fill="#f5f2ec">opencommunications.consulting</text>
+  <text x="1120" y="602" text-anchor="end" font-family="'DM Sans', system-ui, sans-serif" font-size="17" letter-spacing="3" fill="#9aa3b2">CHANGE THAT STICKS · CANADA</text>
+</svg>