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/workspace_only.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+# Narrows the workspace in ./pubspec.yaml to the members named, for an image that builds
+# only those (the Dockerfiles of Learn's server and curator): pub then resolves without
+# the other members, whose folders the image doesn't hold. Fails if one named isn't a
+# member, so a renamed package fails the build loudly.
+#
+#   sh tool/workspace_only.sh apps/learn/halkyon_learn_server packages/halkyon_base_server
+set -eu
+awk -v keep="$*" '
+  BEGIN { n = split(keep, k, " "); for (i = 1; i <= n; i++) wanted["  - " k[i]] = 1 }
+  /^workspace:/ { block = 1; print; next }
+  block && /^  - / { if ($0 in wanted) { print; found[$0] = 1 }; next }
+  { block = 0; print }
+  END {
+    for (w in wanted) if (!(w in found)) { print "Not a workspace member:" substr(w, 4) > "/dev/stderr"; failed = 1 }
+    exit failed
+  }
+' pubspec.yaml >pubspec.yaml.narrowed
+mv pubspec.yaml.narrowed pubspec.yaml