build: Melos, a Dart check and every path for the single repository

- Melos 8.9 as the root's dev dependency, with scripts across the
  workspace (analyze, test:dart, test:flutter, generate, check) and its
  filters (--diff, --include-dependents); cli_util overridden to 0.5.2
  for it, since Learn's flutter_launcher_icons asks for 0.4 and uses only
  its logger, unchanged in 0.5.
- tool/check.dart in place of tool/check.sh: packages, formatting of our
  own files, analysis, the tools' tests, then every package's tests
  through Melos (--changed-since=<commit> for only what changed and what
  depends on it). tool/pre-push is the hook: the signature check, then
  the checks. CI generates every server's code and runs the same check.
- The images build from the root with a workspace narrowed to what they
  need (tool/workspace_only.sh); compose, the deploy's migration test,
  the snap, the release script, the editor's launch files and Learn's
  asset tools (run from apps/learn) follow the new layout.
- The project's source is named as project-halkyon/halkyon (source link,
  snap, F-Droid listing, release uploads); the AGPL packages carry the
  licence's own text.

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

apps/learn/halkyon_learn_server/lib/src/auth/learn_base.dart

@@ -7,7 +7,7 @@ import 'package:halkyon_learn_server/src/instance/app_compatibility.dart';
 /// to the people who use the instance (AGPL-3.0-or-later for the app,
 /// SSPL-1.0 for the server).
 const upstreamSourceCodeUrl =
-    'https://code.halkyon.app/project-halkyon/halkyon_learn';
+    'https://code.halkyon.app/project-halkyon/halkyon';
 
 /// How Halkyon Learn sets halkyon_base up (ADR 0014), the module giving every
 /// Halkyon server its sign-in, the account and its deletion, the pruning and