chore(lint): LeanCode's strict rules and a pre-push check

The workspace root's analysis_options.yaml includes leancode_lint 27 and enables its analyzer plugin (rules for blocs, Equatable and Flutter layout); the server and the app include that file, while the generated client keeps its own options. Plugins can only be enabled at the root of a pub workspace and only reach packages whose options include the root file, which was verified before settling on this layout. Most rules are warnings, casts, inference and raw types are strict, and the check treats infos as fatal too. lints and flutter_lints are no longer needed.

tool/check.sh checks formatting, runs dart analyze --fatal-infos and the server tests; .git/hooks/pre-push links to it, so every push has to pass it. AGENTS.md describes the checklist and how to install the hook in a fresh clone.

Fixes for the new rules: package imports, run() returns Future<void>, the server setup as one cascade, a primary constructor for the app widget, a const and a redundant argument. The workspace root keeps its conventional name `_` with a documented ignore.

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

pubspec.yaml

@@ -1,4 +1,6 @@
-name: _
+# `_` is the conventional name of a pub workspace's root, which is never published or
+# imported, so the naming lint doesn't apply to it.
+name: _ # ignore: package_names
 publish_to: none
 
 environment:
@@ -9,3 +11,6 @@ workspace:
   - halkyon_learn_server
   - halkyon_learn_flutter
 
+dev_dependencies:
+  # Referenced by analysis_options.yaml in this directory.
+  leancode_lint: ^27.0.0