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/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)
+```