feat: add mix setup/git_hooks, activate a pre-push conventional-commit hook

mix git_hooks sets core.hooksPath to githooks/ (idempotent - setting the
same git config value repeatedly is a no-op). mix setup runs it, and is
expected to grow more steps over time as more repo-management tasks land
in this project.

Adds a new pre-push hook alongside the existing commit-msg one - it
reuses ci/conventional_commits.sh as-is (no changes needed: with no env
vars set it already falls back to main/origin/main as base and the
current branch as head, exactly right for a local push) to catch
anything that slipped past commit-msg (a commit made before the hooks
were installed, an amend, etc.), not just the current HEAD.

Removes app/lib/mix/tasks/githooks.install.ex - an exact duplicate of
this same idea living in app/'s own mix project instead of the new
top-level one. Updates Readme.adoc/AGENTS.md/app/usage-rules.md's
references to the old manual `git config core.hooksPath githooks` /
`mix githooks.install` steps to point at `mix setup` instead.

Author: bougyman

AGENTS.md

@@ -14,9 +14,9 @@ and best practices for agents to follow.
 
 ## Standards
 
-- Conventional Commits: app/usage-rules.md — enforced by the `commit-msg` hook
-  at `githooks/commit-msg` (run `git config core.hooksPath githooks` once per
-  clone to activate it).
+- Conventional Commits: app/usage-rules.md — enforced by the `commit-msg`
+  and `pre-push` hooks at `githooks/` (run `mix setup` once per clone to
+  activate them).
 
 <!-- usage-rules-start -->
 <!-- ash-start -->

Readme.adoc

@@ -331,12 +331,11 @@ $ lproj list --mine
 == Development
 
 First, activate the repo's git hooks (enforces conventional-commit subjects
-before you even push):
+on every commit, and again on every commit about to be pushed):
 
 [source,sh]
 ----
-$ cd app
-$ mise exec -- mix githooks.install
+$ mix setup
 ----
 
 The project uses ExUnit and `mix format`. Run tests with:

app/lib/mix/tasks/githooks.install.ex

@@ -7,9 +7,12 @@
 - Use the imperative, present tense in the description (`add`, not `added`/`adds`).
 - Mark breaking changes with `!` before the colon (e.g. `feat!: ...`).
 - Bare `Merge branch ...` subjects are rejected — reword as `chore: Merge branch ...`.
-- Enforced locally by the `commit-msg` hook at `githooks/commit-msg`, which
-  delegates to `ci/validate_conventional_commit.sh` — run
-  `git config core.hooksPath githooks` once per clone to activate it.
-- Enforced in CI across a whole PR's commit range by `ci/conventional_commits.sh`
-  (same validator, run per-commit; skips GitHub's own auto-generated
-  update-branch merge commits).
+- Enforced locally by the `commit-msg` hook at `githooks/commit-msg` (each
+  commit's own subject, via `ci/validate_conventional_commit.sh`) and the
+  `pre-push` hook at `githooks/pre-push` (every commit about to be pushed,
+  via `ci/conventional_commits.sh` - catches anything that slipped past
+  `commit-msg`, e.g. a commit made before the hooks were installed) — run
+  `mix setup` once per clone to activate both.
+- Enforced in CI across a whole PR's commit range by the same
+  `ci/conventional_commits.sh` the `pre-push` hook uses (skips GitHub's own
+  auto-generated update-branch merge commits).

app/usage-rules.md

@@ -0,0 +1,9 @@
+#!/bin/sh
+# Enforces Conventional Commits on every commit about to be pushed, not
+# just whatever commit-msg already checked at commit time (catches a
+# commit made before this hook was installed, an amend, a rebase, etc.).
+# See app/usage-rules.md for the rule. Activate with:
+#   git config core.hooksPath githooks
+
+repo_top=$(git rev-parse --show-toplevel) || exit 1
+exec "$repo_top/ci/conventional_commits.sh"

githooks/pre-push

@@ -0,0 +1,28 @@
+defmodule Mix.Tasks.GitHooks do
+  @shortdoc "Installs this repo's git hooks (commit-msg, pre-push - Conventional Commits)"
+
+  @moduledoc """
+  #{@shortdoc}.
+
+      mix git_hooks
+
+  Sets `core.hooksPath` to `githooks/` (this repo's own `commit-msg` and
+  `pre-push` hooks, both enforcing Conventional Commits via
+  `ci/validate_conventional_commit.sh`/`ci/conventional_commits.sh`) -
+  the same one-line `git config` this repo's docs already told you to run
+  by hand, just idempotent and easy to re-run. Safe to run repeatedly:
+  setting the same git config value twice is a no-op. Wired into
+  `mix setup` - see that task.
+  """
+
+  use Mix.Task
+
+  alias RepoTasks.Shell
+
+  @impl Mix.Task
+  def run(_argv) do
+    Shell.run!("git", ["config", "core.hooksPath", "githooks"])
+    Mix.shell().info("==> Git hooks installed (core.hooksPath = githooks)")
+    :ok
+  end
+end

lib/mix/tasks/git_hooks.ex

@@ -0,0 +1,21 @@
+defmodule Mix.Tasks.Setup do
+  @shortdoc "Sets up this repo for development"
+
+  @moduledoc """
+  #{@shortdoc}.
+
+      mix setup
+
+  Runs every one-time/idempotent setup step this repo needs. Currently
+  just `mix git_hooks` - expected to grow (e.g. app/'s own `mix deps.get`)
+  as more repo-management tasks land here.
+  """
+
+  use Mix.Task
+
+  @impl Mix.Task
+  def run(_argv) do
+    Mix.Task.run("git_hooks")
+    :ok
+  end
+end