test(coverage): add targeted tests to raise per-package coverage gates

- internal/homesick/version: new version_test.go covers String constant
  and semver format validation
- internal/homesick/cli: add list, generate, clone, status, diff, and
  git-repo helper tests; coverage raised from 62.5% to 71.2%
- internal/homesick/core: new helpers_test.go covers runGit pretend,
  actionVerb, sayStatus, unlinkPath, linkPath, readSubdirs,
  matchesIgnoredDir, confirmDestroy, ExecAll edge cases, and
  Link/Unlink default castle wrappers; core_test.go and pull_test.go
  extended with New constructor and PullAll quiet-mode tests;
  exec_test.go extended with ExecAll no-repos-dir and error-wrap tests;
  coverage raised from 75.6% to 80.2%

internal/homesick/core/core_test.go

@@ -2,6 +2,7 @@ package core
 
 import (
 	"bytes"
+	"path/filepath"
 	"testing"
 )
 
@@ -47,3 +48,29 @@ func TestDeriveDestination(t *testing.T) {
 		})
 	}
 }
+
+func TestNewInitializesApp(t *testing.T) {
+	stdout := &bytes.Buffer{}
+	stderr := &bytes.Buffer{}
+
+	app, err := New(stdout, stderr)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if app == nil {
+		t.Fatal("expected app instance")
+	}
+
+	if app.Stdout != stdout {
+		t.Fatal("expected stdout writer to be assigned")
+	}
+	if app.Stderr != stderr {
+		t.Fatal("expected stderr writer to be assigned")
+	}
+	if app.HomeDir == "" {
+		t.Fatal("expected home directory to be set")
+	}
+	if app.ReposDir != filepath.Join(app.HomeDir, ".homesick", "repos") {
+		t.Fatalf("unexpected repos dir: %q", app.ReposDir)
+	}
+}