Skip vaultik's own index and PID file in a backup (closes #421)

A backup of a directory that holds the local index stored a torn copy
of it, its -wal and -shm side files and vaultik.pid, all being written
while they were read. The scan now treats these files as excluded,
whatever the config says. Their paths come from index_path and the lock
directory, with symlinks in their directories resolved the way source
paths are. If index_path is itself a symlink, SQLite writes the file it
points to, so that file and its side files are skipped too. pidlock now
owns the lock directory and the PID file name, so the CLI and the
scanner use the same path.

Model: opus-5-5

internal/cli/entry_snapshot_create_own_files_test.go

@@ -16,8 +16,7 @@ import (
 // pattern set. The summary counts only the other file. See
 // https://git.eeqj.de/sneak/vaultik/issues/421.
 //
-// Not parallel: it replaces os.Args, os.Stdout, os.Stderr and the xdg
-// globals.
+//nolint:paralleltest // replaces os.Args, os.Stdout, os.Stderr and the xdg globals
 func TestEntrySnapshotCreateSkipsOwnFiles(t *testing.T) {
 	for _, testCase := range []struct {
 		name string