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>

tool/pre-push

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+# The pre-push hook: commits going to main must be signed as deploying it requires
+# (tool/check_signatures.sh, a moment), then the checks CI runs (tool/check.dart,
+# minutes). A push only goes through when both pass.
+#
+# Install it in a fresh clone:  ln -s ../../tool/pre-push .git/hooks/pre-push
+# Skip it for one push:         git push --no-verify
+set -euo pipefail
+cd "$(git rev-parse --show-toplevel)"
+
+# As a hook in a worktree, git hands this its GIT_DIR, an absolute path to the worktree's
+# folder in the main repository. pub runs git with -C on its git dependencies' caches, which
+# GIT_DIR overrides, so it judged the workspace's resolution stale, resolved it with dart
+# alone and failed ("halkyon_learn_flutter requires the Flutter SDK"; 2026-10-16). Nothing
+# below needs them: git finds the repository from the folder it runs in.
+unset GIT_DIR GIT_WORK_TREE GIT_INDEX_FILE GIT_COMMON_DIR GIT_OBJECT_DIRECTORY GIT_PREFIX
+
+# git's arguments (the remote) and stdin (the refs pushed) go to the signature check.
+tool/check_signatures.sh "$@"
+exec dart run tool/check.dart