feat: add mix githooks.install to activate the repo's git hooks (#51)

## Summary

Closes #50.

`githooks/commit-msg` exists and correctly calls
`ci/validate_conventional_commit.sh`, but `core.hooksPath` was never
actually configured on any local clone - the hook never fired, which is
how `fix!(docs)`/`docs!(licence)` (wrong `!` position) got committed
without ever being caught before push.

Adds `mix githooks.install` (`git config core.hooksPath githooks`) and
mentions it in the Readme's Development section so it's actually
discoverable.

Related: `main` now also requires the `Test` and `Validate Commit
Subjects` checks to pass before merge, with no admin override - so even
if the local hook is skipped, CI backstops it.

## Test plan

- [x] `mix test` - 182 passed
- [x] `mix format --check-formatted` - clean
- [x] Ran `mix githooks.install` for real, then fed a malformed subject
(`fix!(docs): ...`) through the now-active `githooks/commit-msg` hook
directly - correctly rejected
- [x] `asciidoctor -o /dev/null Readme.adoc` renders clean

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

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

Author: bougyman

Readme.adoc

@@ -265,6 +265,15 @@ $ lproj list --mine
 
 == Development
 
+First, activate the repo's git hooks (enforces conventional-commit subjects
+before you even push):
+
+[source,sh]
+----
+$ cd app
+$ mise exec -- mix githooks.install
+----
+
 The project uses ExUnit and `mix format`. Run tests with:
 
 [source,sh]

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

@@ -0,0 +1,21 @@
+defmodule Mix.Tasks.Githooks.Install do
+  @moduledoc "Activates this repo's githooks/ (conventional-commit enforcement on commit-msg)."
+  @shortdoc "Activates the repo's git hooks"
+
+  use Mix.Task
+
+  @impl Mix.Task
+  def run(_args) do
+    with {root, 0} <- System.cmd("git", ["rev-parse", "--show-toplevel"], stderr_to_stdout: true),
+         root = String.trim(root),
+         {_output, 0} <-
+           System.cmd("git", ["config", "core.hooksPath", "githooks"],
+             cd: root,
+             stderr_to_stdout: true
+           ) do
+      Mix.shell().info("githooks activated (core.hooksPath = githooks)")
+    else
+      {output, _status} -> Mix.raise("Failed to activate githooks: #{String.trim(output)}")
+    end
+  end
+end