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

docs/README.md

@@ -4,19 +4,22 @@ Where to look for what:
 
 | Document | What it is |
 |---|---|
+| [`../CONTRIBUTING.md`](../CONTRIBUTING.md) | How to contribute: issue-first workflow, which branch to target, translations, the rules a change is reviewed against |
+| [`BUILDING.md`](BUILDING.md) | Building from source: submodule, JDK/SDK requirements, Gradle tasks, what CI runs |
 | [`ARCHITECTURE.md`](ARCHITECTURE.md) | Orientation tour: principles, layers, navigation, recurring-write / conflict / reminder pipelines, testing |
+| [`RELEASING.md`](RELEASING.md) | Release process: versioning, the merge-driven pipeline, the two-forge split, secrets, key custody |
 | [`../CHANGELOG.md`](../CHANGELOG.md) | Release history (Keep a Changelog, SemVer) |
-| [`../.planning/ROADMAP.md`](../.planning/ROADMAP.md) | Living roadmap: shipped milestones, current scope, idea backlog |
-| [`../.planning/PROJECT.md`](../.planning/PROJECT.md) | What the project is, stack, naming, infrastructure |
-| [`../.planning/REQUIREMENTS.md`](../.planning/REQUIREMENTS.md) | Requirement checklist per milestone |
-| [`../.planning/STATE.md`](../.planning/STATE.md) | Snapshot of where development currently stands |
-| [`superpowers/specs/`](superpowers/specs/) | The original design spec (2026-06-08) — historical record, not updated |
-| [`superpowers/plans/`](superpowers/plans/) | Per-milestone implementation plans with task checklists — historical record of how each slice was built, including provider lessons learned |
+| [Issues](https://codeberg.org/jlmakiola/calendula/issues) + [milestones](https://codeberg.org/jlmakiola/calendula/milestones) | **The roadmap.** What's planned, in progress, and shipped — a milestone maps to its `release/vX.Y.Z` branch |
+| [`../.planning/PROJECT.md`](../.planning/PROJECT.md) | What the project is: core value, stack + version pins, constraints, naming, forge/release infrastructure |
+| [`design/`](design/) | Per-feature design notes kept for features whose provider behaviour is worth recording |
 | [`../fastlane/metadata/android/`](../fastlane/metadata/android/) | Store metadata (single source of truth): descriptions, title, icon, screenshots (DE + EN). Harvested directly by the official F-Droid repo; transformed into the self-hosted repo layout at release time by [`../scripts/fastlane_to_fdroid_localized.sh`](../scripts/fastlane_to_fdroid_localized.sh) |
 | [`../fdroid-metadata/`](../fdroid-metadata/) | App-level F-Droid control file (`*.yml`: Categories, License, links) for the self-hosted repo's `fdroid update` |
-| [`fdroid-official/`](fdroid-official/) | Draft recipe + notes for publishing to the **official** F-Droid repo (reproducible build + developer-signed binary) |
+| [`fdroid-official/`](fdroid-official/) | Recipe + notes for publishing to the **official** F-Droid repo (reproducible build + developer-signed binary) |
 
-Conventions: plans and specs under `superpowers/` are point-in-time
-artifacts of the agentic workflow that built each milestone — they get
-status updates but are never rewritten. The `.planning/` files are living
-documents and should stay current.
+Conventions: planning lives in the **issue tracker**, not in this repository. The
+`.planning/` files that predated it (a roadmap, a development-state snapshot, and
+a per-milestone requirement checklist) are gone — issues and milestones say the
+same thing without going stale. `PROJECT.md` is what remains, and it describes
+the project rather than its plan.
+`ARCHITECTURE.md` is the authoritative orientation tour: it is updated with the
+code, and is the right place for a lesson learned about the calendar provider.