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>

.vscode/launch.json

@@ -4,7 +4,7 @@
     // If the server pubspec configures multiple Flutter apps, add an attach
     // configuration like this for each app and add every configuration name
     // to the compound below. Use the VM-service info file pattern
-    // `${workspaceFolder}/halkyon_learn_server/.dart_tool/serverpod/flutter-vm-service-info-<appId>.json`,
+    // `${workspaceFolder}/apps/learn/halkyon_learn_server/.dart_tool/serverpod/flutter-vm-service-info-<appId>.json`,
     // where `<appId>` is the app's key under `serverpod.flutter_apps` in the
     // server pubspec.
     {
@@ -13,7 +13,7 @@
       "name": "halkyon_learn_flutter",
       "type": "dart",
       "request": "attach",
-      "vmServiceInfoFile": "${workspaceFolder}/halkyon_learn_server/.dart_tool/serverpod/flutter-vm-service-info-halkyon_learn.json",
+      "vmServiceInfoFile": "${workspaceFolder}/apps/learn/halkyon_learn_server/.dart_tool/serverpod/flutter-vm-service-info-halkyon_learn.json",
       "preLaunchTask": "serverpod_start",
       "internalConsoleOptions": "neverOpen"
     },
@@ -21,7 +21,7 @@
       "name": "halkyon_learn_server",
       "type": "dart",
       "request": "attach",
-      "vmServiceInfoFile": "${workspaceFolder}/halkyon_learn_server/.dart_tool/serverpod/vm-service-info.json",
+      "vmServiceInfoFile": "${workspaceFolder}/apps/learn/halkyon_learn_server/.dart_tool/serverpod/vm-service-info.json",
       "preLaunchTask": "serverpod_start",
       "internalConsoleOptions": "neverOpen"
     }

.vscode/tasks.json

@@ -7,7 +7,7 @@
       "command": "serverpod",
       "args": ["--no-analytics", "start"],
       "options": {
-        "cwd": "${workspaceFolder}/halkyon_learn_server",
+        "cwd": "${workspaceFolder}/apps/learn/halkyon_learn_server",
         "env": {
           // Marks a `serverpod start` launched from the IDE, which is the
           // only kind that honors the Flutter device picked in VS Code. A run