Halkyon's shared packages

A pub workspace of the Dart packages Halkyon's apps share: halkyon_ui,
Halkyon's look (the theme, fonts, layout and shared widgets, with texts
in English and German), halkyon_core's low-level primitives and
halkyon_interop's cross-app contracts. Dart 3.13 with LeanCode's lints,
configured once at the root; apps take a package by git, pinned to a
signed commit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

CONTRIBUTING.md

@@ -0,0 +1,19 @@
+# Contributing to halkyon_shared
+
+Thanks for your interest in contributing.
+
+## License
+
+By contributing, you agree that your contributions will be licensed under
+AGPL-3.0-or-later. See LICENSE.
+
+## Developer Certificate of Origin
+
+All commits must be signed off per the Developer Certificate of Origin.
+Add `-s` to your git commit command, or set `git config format.signoff true`.
+
+## Pull requests
+
+- Target `main`.
+- Keep PRs focused.
+- Tests must pass in CI.