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 -->
+