Add build hook system

Hook scripts in .slam/hooks/ are executed at each stage of the build
pipeline. If a script exists and is executable, it runs; a non-zero
exit stops the build.

Available hooks:
  pre_format / post_format
  pre_install / post_install
  pre_test / post_test
  pre_build / post_build

The init command now creates .slam/hooks/ with a README documenting
usage and examples, and includes a default pre_format hook that
initializes any git submodules that are present.

test/test_slam.py

@@ -45,3 +45,89 @@ def test_install(monkeypatch):
 def test_test(monkeypatch):
     monkeypatch.setattr("poetry_slam.build_tool.subprocess", result_factory(out=b"out", err=b"err", code=0))
     assert BuildTool(verbose=True).test([]) == 0
+
+
+def test_run_hook_executes_script(monkeypatch, tmp_path):
+    """Hook script exists and is executable — it should be run."""
+    hooks_dir = tmp_path / ".slam" / "hooks"
+    hooks_dir.mkdir(parents=True)
+    hook = hooks_dir / "pre_build"
+    hook.write_text("#!/bin/sh\nexit 0\n")
+    hook.chmod(0o755)
+
+    mock_sub = result_factory()
+    monkeypatch.setattr("poetry_slam.build_tool.subprocess", mock_sub)
+    bt = BuildTool(project_root=tmp_path)
+    bt.run_hook("pre_build")
+    mock_sub.run.assert_called_once()
+
+
+def test_run_hook_skips_missing(monkeypatch, tmp_path):
+    """No hook script — run_hook should do nothing."""
+    mock_sub = result_factory()
+    monkeypatch.setattr("poetry_slam.build_tool.subprocess", mock_sub)
+    bt = BuildTool(project_root=tmp_path)
+    bt.run_hook("pre_build")
+    mock_sub.run.assert_not_called()
+
+
+def test_run_hook_non_executable_raises(monkeypatch, tmp_path):
+    """Hook script exists but is not executable — should raise BuildError."""
+    hooks_dir = tmp_path / ".slam" / "hooks"
+    hooks_dir.mkdir(parents=True)
+    hook = hooks_dir / "pre_build"
+    hook.write_text("#!/bin/sh\nexit 0\n")
+    hook.chmod(0o644)
+
+    monkeypatch.setattr("poetry_slam.build_tool.subprocess", result_factory(err=b"permission denied", code=126))
+    bt = BuildTool(project_root=tmp_path)
+    with pytest.raises(BuildError):
+        bt.run_hook("pre_build")
+
+
+def test_run_hook_raises_on_failure(monkeypatch, tmp_path):
+    """Hook script fails — should raise BuildError."""
+    hooks_dir = tmp_path / ".slam" / "hooks"
+    hooks_dir.mkdir(parents=True)
+    hook = hooks_dir / "pre_build"
+    hook.write_text("#!/bin/sh\nexit 1\n")
+    hook.chmod(0o755)
+
+    monkeypatch.setattr("poetry_slam.build_tool.subprocess", result_factory(err=b"hook failed", code=1))
+    bt = BuildTool(project_root=tmp_path)
+    with pytest.raises(BuildError):
+        bt.run_hook("pre_build")
+
+
+def test_build_runs_hooks(monkeypatch, tmp_path):
+    """Build pipeline should call run_hook for each stage."""
+    mock_sub = result_factory()
+    monkeypatch.setattr("poetry_slam.build_tool.subprocess", mock_sub)
+    bt = BuildTool(project_root=tmp_path)
+
+    hook_calls = []
+    original_run_hook = bt.run_hook
+
+    def tracking_run_hook(stage):
+        hook_calls.append(stage)
+        original_run_hook(stage)
+
+    bt.run_hook = tracking_run_hook
+    bt.build()
+
+    assert hook_calls == [
+        "pre_format",
+        "post_format",
+        "pre_install",
+        "post_install",
+        "pre_test",
+        "post_test",
+        "pre_build",
+        "post_build",
+    ]
+
+
+def test_hooks_dir_property(tmp_path):
+    """hooks_dir should point to .slam/hooks/ under project_root."""
+    bt = BuildTool(project_root=tmp_path)
+    assert bt.hooks_dir == tmp_path / ".slam" / "hooks"