Replace symlinks with plain files containing relative paths

- Remove all symlink creation and resolution in favor of plain files
- currentvault file now contains relative path like "vaults.d/default"
- current-unlocker file now contains relative path like "unlockers.d/passphrase"
- current version file now contains relative path like "versions/20231215.001"
- Simplify path resolution to just read file contents and join with parent dir
- Update all tests to read files instead of using os.Readlink

internal/vault/vault_error_test.go

@@ -84,4 +84,4 @@ func TestAddSecretCleansUpOnFailure(t *testing.T) {
 	secretDir := filepath.Join(vaultDir, "secrets.d", secretName)
 	exists, _ := afero.DirExists(fs, secretDir)
 	assert.False(t, exists, "Secret directory should not exist after failed AddSecret")
-}
+}