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/exec_test.go

@@ -88,3 +88,22 @@ func (s *ExecSuite) TestExec_PretendDoesNotExecuteCommand() {
 	require.NoFileExists(s.T(), target)
 	require.Contains(s.T(), s.stdout.String(), "Would execute")
 }
+
+func (s *ExecSuite) TestExecAll_RequiresCommand() {
+	err := s.app.ExecAll(nil)
+	require.Error(s.T(), err)
+	require.Contains(s.T(), err.Error(), "exec_all requires COMMAND")
+}
+
+func (s *ExecSuite) TestExecAll_NoReposDirIsNoop() {
+	missingRepos := filepath.Join(s.T().TempDir(), "missing", "repos")
+	app := &core.App{
+		HomeDir:  s.homeDir,
+		ReposDir: missingRepos,
+		Stdout:   s.stdout,
+		Stderr:   s.stderr,
+	}
+
+	err := app.ExecAll([]string{"echo hi"})
+	require.NoError(s.T(), err)
+}