docs: add a contributing guide

Codeberg became canonical for issues, PRs and releases, but there was
nothing telling a contributor how any of it works — no CONTRIBUTING.md at
all, and no contributor entry point in the README.

The workflow it documents is issue-first: a feature needs a yes-or-no
before it needs code, since whether Calendula should do a thing is the one
decision a patch cannot make. Both features and bugs get a milestone, and
that milestone names the branch a PR targets (2.18.0 -> release/v2.18.0),
because main is a release trigger rather than a staging area. Typo and docs
fixes skip straight to a PR — an issue-per-typo helps nobody.

Two traps get their own sections because both waste a contributor's whole
afternoon: translations never go through PRs (Weblate owns every values-*
file, German included, and overwrites hand-edits on the next sync), and a
clone without --recurse-submodules cannot configure at all, since
floret-kit is a composite build compiled from source.

The rules section is the invariants that actually turn into review
comments — no network, no second database, no hand-patching UI state after
a write, domain/ free of Android imports, JVM-first tests, and the
reproducible-build flags that the official F-Droid repo depends on.

.forgejo/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,30 @@
+---
+name: Bug report
+about: Something doesn't work the way it should
+title: ""
+labels:
+  - bug
+---
+
+### What happened
+
+
+### What you expected
+
+
+### Steps to reproduce
+1.
+2.
+3.
+
+### Environment
+- Calendula version: <!-- Settings → bottom of the screen -->
+- Android version:
+- Device:
+- Installed from: <!-- official F-Droid / the self-hosted repo / built from source -->
+- Affected calendar: <!-- Google, CalDAV (DAVx5, Nextcloud, …), on-device/local,
+                         subscribed/WebCal, birthdays — provider behaviour differs
+                         a lot per account type, so this often points straight at
+                         the cause -->
+- Time zone: <!-- only if the problem involves dates or all-day events -->
+

.forgejo/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,18 @@
+# Kept enabled so anything that doesn't fit the four templates still has a way
+# in (the `ToDo` label exists for exactly those).
+blank_issues_enabled: true
+
+contact_links:
+  - name: Translate Calendula
+    url: https://weblate.dev.jeanlucmakiola.de/engage/calendula/
+    about: >-
+      Translations are managed on Weblate, not here — it owns every values-*
+      file, so a hand-edited translation gets overwritten on the next sync.
+      No coding needed: pick or request a language and translate in the browser.
+
+  - name: Contributing guide
+    url: https://codeberg.org/jlmakiola/calendula/src/branch/main/CONTRIBUTING.md
+    about: >-
+      Before opening a pull request: the issue-first workflow, which release
+      branch to target, how to build (there's a submodule), and the
+      architectural rules a change is reviewed against.

.forgejo/ISSUE_TEMPLATE/crash_report.md

@@ -0,0 +1,27 @@
+---
+name: Crash report
+about: Report a crash. Calendula can capture this for you (Settings → Report a problem, or the prompt after a crash) — it copies the report to your clipboard and prefills this form.
+title: "Crash: "
+labels:
+  - bug
+  - crash
+  - priority/high
+---
+
+<!--
+Thanks for reporting a crash in Calendula!
+
+If the app prefilled this for you, the crash report is already below — just add
+what you were doing and submit. Otherwise, paste the report from your clipboard
+into the code block. The report contains only app/Android/device versions and the
+stack trace — no personal data or calendar content.
+-->
+
+### What happened
+
+
+### Crash report
+
+```
+(paste the crash report here)
+```

.forgejo/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,16 @@
+---
+name: Feature request
+about: Suggest an idea or improvement
+title: ""
+labels:
+  - feat
+---
+
+### What would you like Calendula to do?
+
+
+### Why — what problem does it solve?
+
+
+### Anything else
+<!-- mockups, examples from other apps, alternatives you considered -->

.forgejo/ISSUE_TEMPLATE/question.md

@@ -0,0 +1,19 @@
+---
+name: Question
+about: Ask how something works or get help using Calendula
+title: ""
+labels:
+  - question
+---
+
+### Your question
+
+
+### What you've tried
+<!-- so far, if anything -->
+
+
+### Context
+- Calendula version: <!-- Settings → bottom of the screen -->
+- Android version:
+- Device: