fix(coding-agent): render developer-role messages in /export and /tree

The HTML export feed, sidebar tree, and TUI session tree did not handle
the developer role, so plan content injected after /plan approval (and
any other developer messages) was hidden from /export and shown as a
bare [developer] label in /tree.

Renders developer messages in the main feed with a dimmed
.developer-message style, labels them in the sidebar tree, counts them
in header stats, and shows their content in the TUI tree selector so
search matches the body.

Closes #753.

Co-Authored-By: omp <noreply@oh-my-pi.dev>

packages/coding-agent/src/export/html/template.css

@@ -179,6 +179,10 @@
       color: var(--accent);
     }
 
+    .tree-role-developer {
+      color: var(--dim);
+    }
+
     .tree-role-assistant {
       color: var(--success);
     }
@@ -316,6 +320,14 @@
       position: relative;
     }
 
+    .user-message.developer-message {
+      opacity: 0.7;
+    }
+
+    .user-message.developer-message .markdown-content {
+      color: var(--dim);
+    }
+
     .assistant-message {
       padding: 0;
       position: relative;