Model rules: who a chat's model may bring into a conversation

Rules read from the main model decide who it is offered as a crowd member,
a friend and on its roster; any-to-any with denies by default, or
none-to-none with allows. The crowd picker names what it holds back and
why, and a model held back only by a person's own rule -- or by anything,
with the new rules.override -- can still be added by hand. Another data
group is now a deny that an explicit rule opens. Admin -> Model rules and
a card in Settings, each with a matrix drawn by the enforcing function.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lembas/web/templates/settings.html

@@ -185,6 +185,37 @@
               </ul>
             </div>
             {% endif %}
+            {#
+              Who your models may talk to. Anybody may narrow the instance's rules
+              for themselves; widening them takes `rules.override`, and the text
+              below says which of the two this person has, so a rule that does
+              nothing is not a mystery.
+            #}
+            <div class="card">
+              <h2 class="card__title">{{ t("Who your models may talk to") }}</h2>
+              <p class="card__lede">
+                {% if talk_override %}
+                {{ t("Your rules and your setting win over the instance's, for you. You can also add any model to a crowd by hand.") }}
+                {% else %}
+                {{ t("Your rules can only narrow the instance's. A model your own rule holds back can still be added to a crowd by hand; one the instance holds back cannot.") }}
+                {% endif %}
+              </p>
+              <form method="post" action="/api/preferences/talk-mode" class="row"
+                    style="gap: var(--sp-2); margin-bottom: var(--sp-4)">
+                <label class="visually-hidden" for="talk-mode">{{ t("Your starting point") }}</label>
+                <select class="select" id="talk-mode" name="mode" style="flex: 1; min-width: 0">
+                  <option value="" {{ 'selected' if not talk_mode }}>{{ t("Follow the instance") }}</option>
+                  <option value="open" {{ 'selected' if talk_mode == 'open' }}>{{ t("Any model may talk to any other") }}</option>
+                  <option value="closed" {{ 'selected' if talk_mode == 'closed' }}>{{ t("No model may talk to another") }}</option>
+                </select>
+                <button class="btn" type="submit">{{ t("Save") }}</button>
+              </form>
+              {% set rules_action = "/api/preferences/talk-rules" %}
+              {% set rules = talk_rules %}
+              {% set matrix = talk_matrix %}
+              {% set matrix_models = talk_matrix_models %}
+              {% include "partials/_talk_rules.html" %}
+            </div>
           </section>
 
           {# --- Appearance --- #}