Write a contributing guide, and fix what the forge move left stale (#96)

(docs) Update all the docs to be up to date with current status quo

.planning/PROJECT.md

@@ -17,9 +17,10 @@ re-inventing the calendar sync stack — leave that to DAVx5 and the system.
 ## Current Milestone
 
 Milestones 1 (read, v1.0) and 2 (write support, v1.1–v2.0.0 incl. reminder
-delivery) are **complete** — v2.0.0 shipped 2026-06-11. Next is v3.0
-(power-user features) plus an undecided "Locations & People" idea backlog;
-see `ROADMAP.md`.
+delivery) are **complete** — v2.0.0 shipped 2026-06-11. Everything since is
+tracked as issues and milestones on Codeberg:
+<https://codeberg.org/jlmakiola/calendula/milestones>. A milestone maps to its
+`release/vX.Y.Z` branch.
 
 ## Stack
 
@@ -27,10 +28,26 @@ Kotlin 2.3.21 (paired with KSP 2.3.9 — Kotlin 2.4.0 has no KSP release
 yet, do not upgrade until one ships). Jetpack Compose + Material 3
 Expressive 1.5.0-alpha21 (alpha is intentional — Expressive APIs only
 live in the 1.5 alpha line). Hilt 2.59.2, DataStore. Gradle Kotlin DSL
-with Version Catalog. AGP 9.1.1, Gradle 9.5.1. JVM target 17.
+with Version Catalog. AGP 9.2.1, Gradle 9.5.1. JVM target 17 (exactly — AGP
+requires it).
 
-Android-only (minSdk 29, targetSdk 36). No iOS. No `INTERNET` permission —
-any feature that would need one is an explicit product decision first.
+The shared Material 3 Expressive kit, **floret-kit**, is a git submodule wired
+in as a Gradle composite build, so it compiles from source rather than resolving
+as a dependency.
+
+## Constraints
+
+- **Platform:** Android-only, Android 10+ (minSdk 29), targetSdk 36. No iOS.
+- **Offline-first:** all data lives in `CalendarContract` — no app database, no
+  sync stack. No `INTERNET` permission; any feature needing one is an explicit
+  product decision first.
+- **Privacy:** zero telemetry, zero analytics.
+- **i18n:** German + English from day one; further languages come from community
+  translators via the self-hosted Weblate, which owns every `values-*` file.
+- **Tests + CI from day one**, JVM-first.
+- **Reproducible release builds**, so the official F-Droid repo can verify the
+  published binary against a from-source rebuild.
+- **Licence:** MIT.
 
 ## Naming
 
@@ -40,5 +57,9 @@ 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`.
+**Codeberg (`jlmakiola/calendula`) is canonical** — git, issues, PRs, tags and
+releases, plus contributor CI. The self-hosted Gitea instance is build
+infrastructure only: it holds the signing key, runs the release pipeline, and
+publishes the self-hosted F-Droid repo on Hetzner. Codeberg push-mirrors `main`
+and tags to Gitea, where a bumped `versionName` triggers the release. Also
+published to the official F-Droid repo. See `docs/RELEASING.md`.