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>

snap/snapcraft.yaml

@@ -37,7 +37,7 @@ description: |
 
   Questions and reports: support@halkyon.app
 license: AGPL-3.0-or-later
-source-code: https://code.halkyon.app/project-halkyon/halkyon_learn
+source-code: https://code.halkyon.app/project-halkyon/halkyon
 # The project's, not an instance's: the app works with any instance.
 website: https://halkyon.app/learn
 # No contact or issues: the Store refuses an upload whose links aren't http(s) URLs
@@ -127,7 +127,7 @@ parts:
     override-build: |
       set -eu
       export PATH="$CRAFT_STAGE/usr/bin:$PATH"
-      cd halkyon_learn_flutter
+      cd apps/learn/halkyon_learn_flutter
       craftctl set version="$(sed -n 's/^version: *\([^+]*\).*/\1/p' pubspec.yaml)"
       flutter pub get --enforce-lockfile
       flutter build linux --release \