Rename app Floret → Agendula

Floret is promoted to the family / design-language (shared-kit) name; the
tasks app itself becomes Agendula (de.jeanlucmakiola.agendula) — agenda
('things to be done') + Calendula's -ula, a twin of the Calendula name.

Renames the package, namespace, applicationId, rootProject.name, app_name,
FloretApp/FloretNavHost/FloretTransitions classes, theme, F-Droid metadata
dir, CI artifact name, and docs. The botanical word 'florets' is preserved in
the name-origin prose, which is rewritten to Agendula's etymology. Clean
build + unit tests green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/res/drawable/ic_launcher_foreground.xml

@@ -1,10 +1,10 @@
 <?xml version="1.0" encoding="utf-8"?>
 <!--
-    Floret launcher icon foreground — PLACEHOLDER.
+    Agendula launcher icon foreground — PLACEHOLDER.
 
     A simple rounded check mark inside the 108dp adaptive-icon canvas
     (72dp safe zone). Deliberately not Calendula's calendar mark, so the two
-    apps never look alike. Replace with real branding (a stylized floret)
+    apps never look alike. Replace with real branding (a stylized agendula)
     when the design lands — see docs/PLAN.md §9.
 -->
 <vector xmlns:android="http://schemas.android.com/apk/res/android"