docs: add .planning/ project-tracking documents

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.planning/PROJECT.md

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+# Calendula
+
+## What This Is
+
+A modern Material 3 Expressive Android calendar app, read-only V1. Lives
+entirely on top of Android's `CalendarContract` — any calendar synced to the
+device (CalDAV via DAVx5, Google, local, WebCal, …) shows up automatically.
+The differentiator is visual: real Material 3 Expressive design that no
+existing FOSS calendar app delivers.
+
+## Core Value
+
+A calendar app that is genuinely pleasant to look at and use, without
+re-inventing the calendar sync stack — leave that to DAVx5 and the system.
+
+## Current Milestone
+
+**v0.1 — Foundation & CI:** Buildable Android project scaffold with theme,
+icon, i18n, Hilt, DataStore, green CI.
+
+## Stack
+
+Kotlin 2.3, Jetpack Compose + Material 3 Expressive (alpha for 1.5 APIs),
+Hilt, DataStore. Gradle Kotlin DSL with Version Catalog. AGP 9.1, Gradle 9.5.
+
+Read-only V1, write support V2.
+
+Android-only (minSdk 29, targetSdk 36). No iOS.
+
+## Naming
+
+"Calendula" — Latin *kalendae* ("first day of the month", root of "calendar")
+is also the etymological root of the marigold flower Calendula. The icon
+shows a stylized "1" on a slate squircle.
+
+## Source
+
+Hosted on self-hosted Gitea, released through self-hosted F-Droid repo on
+Hetzner. Same infrastructure as `HouseHoldKeaper`.