feat: add mix ci task for the complete quality gate (#127)

## Summary

- Adds `Mix.Tasks.Ci` (`lib/mix/tasks/ci.ex`) that runs the four quality
gate steps (`deps.get`, `format --check-formatted`, `usage_rules.sync
--check`, `mix test`) sequentially inside `app/`, failing fast on the
first error
- Updates `.github/workflows/ci.yaml` to call `mix ci` instead of the
four individual steps — CI and local `mix ci` now share one code path
- Adds `test/mix/tasks/ci_test.exs` verifying the correct sequence of
commands is invoked (using an injected shell runner, matching the
pattern from `lc_test.exs`)

Closes CRY-49

## Test plan

- [x] `mix compile --warnings-as-errors` passes
- [x] `mix format --check-formatted` passes
- [x] `mix test` passes (12/12, including the new `Mix.Tasks.CiTest`)
- [x] CI workflow updated to call `mix ci` with `working-directory: .`
(repo root), overriding the job's `app/` default

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: bougyman

.github/workflows/ci.yaml

@@ -44,17 +44,12 @@ jobs:
             app/_build
           key: ${{ runner.os }}-mix-${{ hashFiles('app/mix.lock') }}
       -
-        run: mix deps.get
-      -
-        run: mix format --check-formatted
-      -
-        # Catches drift introduced within this PR (e.g. an ash/oban bump
-        # without re-running the sync) - see #79. The reactive auto-fix
-        # for drift from any other source lives in its own workflow
-        # (usage-rules-sync.yaml), not here.
-        run: mix usage_rules.sync --check
-      -
-        run: mix test
+        # mix ci covers deps.get, format --check-formatted,
+        # usage_rules.sync --check (catches dep-bump drift, see #79),
+        # and mix test. Runs from the repo root; cd: app/ is handled
+        # inside the task itself.
+        run: mix ci
+        working-directory: .
 
   conventional_commits:
     if: inputs.skip_commit_validation != true

lib/mix/tasks/ci.ex

@@ -0,0 +1,41 @@
+defmodule Mix.Tasks.Ci do
+  @shortdoc "Runs the complete quality gate against app/"
+
+  @moduledoc """
+  #{@shortdoc}.
+
+      mix ci
+
+  Runs every check `.github/workflows/ci.yaml`'s `test` job runs on a pull
+  request, in the same order, so a green `mix ci` locally predicts a green
+  CI run — and CI itself calls this task, so there's one place to fix if
+  either ever breaks:
+
+    1. `mix deps.get` — ensure deps are present
+    2. `mix format --check-formatted` — code is formatted
+    3. `mix usage_rules.sync --check` — usage rules are in sync with deps
+       (catches drift introduced by a dep bump without re-running the sync;
+       see #79)
+    4. `mix test` — all tests pass
+
+  All steps run inside `app/`.
+  """
+
+  use Mix.Task
+
+  alias RepoTasks.Shell
+
+  @impl Mix.Task
+  def run(argv) do
+    run(argv, &Shell.run!/3)
+  end
+
+  @doc false
+  def run(_argv, shell) do
+    shell.("mix", ["deps.get"], cd: "app")
+    shell.("mix", ["format", "--check-formatted"], cd: "app")
+    shell.("mix", ["usage_rules.sync", "--check"], cd: "app")
+    shell.("mix", ["test"], cd: "app")
+    :ok
+  end
+end

test/mix/tasks/ci_test.exs

@@ -0,0 +1,21 @@
+defmodule Mix.Tasks.CiTest do
+  use ExUnit.Case, async: true
+
+  alias Mix.Tasks.Ci
+
+  test "runs all quality gate steps in order" do
+    caller = self()
+
+    shell = fn cmd, args, opts ->
+      send(caller, {:run, cmd, args, opts})
+      :ok
+    end
+
+    assert :ok = Ci.run([], shell)
+
+    assert_receive {:run, "mix", ["deps.get"], [cd: "app"]}
+    assert_receive {:run, "mix", ["format", "--check-formatted"], [cd: "app"]}
+    assert_receive {:run, "mix", ["usage_rules.sync", "--check"], [cd: "app"]}
+    assert_receive {:run, "mix", ["test"], [cd: "app"]}
+  end
+end