Add planning docs: research, requirements, redesign plan

Research of current state, requirements for the web-first pivot
(pantry polish -> auth -> MCP), and the phased implementation plan.

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

kitchen/templates/kitchen/partials/pantry_expiry_edit.html

@@ -1,17 +0,0 @@
-<tr id="pantry-row-{{ item.id }}">
-  <td>{{ item.ingredient.name }}</td>
-  <td>{{ item.quantity|floatformat:0 }} {{ item.unit }}</td>
-  <td colspan="2">
-    <form style="display: flex; gap: 0.4rem; align-items: center;"
-          hx-post="{% url 'app-pantry-save-expiry' item.id %}"
-          hx-target="#pantry-items"
-          hx-swap="innerHTML">
-      <input type="date" name="expiry_date" value="{{ item.expiry_date|date:'Y-m-d' }}" style="font-size: 0.8rem;">
-      <button type="submit" class="btn btn-primary btn-sm">Save</button>
-      <button type="button" class="btn btn-secondary btn-sm"
-              hx-get="{% url 'app-pantry-cancel-edit' %}"
-              hx-target="#pantry-items"
-              hx-swap="innerHTML">Cancel</button>
-    </form>
-  </td>
-</tr>