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/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 -->