fix(store): don't silently create a second store; restrict store perms to 0700

[mikrub]

Two related problems with how the store directory is created.

1. Silent duplicate stores

--store defaults to a CWD-relative ./store, and the store/client constructors create it unconditionally. So running any command from the wrong directory creates an empty second store instead of reporting the mistake. Every later command run from there reads that empty store and reports zero messages — indistinguishable from data loss. An auth against such a directory is worse: it leaves real session keys somewhere nobody is tracking.

This is not hypothetical. It happened three times in a single working session after I had identified the behaviour and was actively watching for it — twice from running the binary directly instead of through a wrapper, once from a go test run inside the repo. That is why I do not think documentation fixes it.

Only auth may now bring a store into existence; every other command errors with the resolved path:

$ whatsapp-cli chats list
{"success":false,"data":null,"error":"store does not exist: /wrong/dir/store (run 'auth' to create it, or pass --store)"}

2. Permissions

The store directories were created 0755, and SQLite creates the DBs 0644 under the usual umask. whatsapp.db holds the linked-device session keys — read access is account impersonation — and messages.db holds the plaintext message archive with contact names and phone numbers. Both are now 0700, along with the media output directory. Media files themselves were already 0600 via CreateTemp+Rename; the directory was the only part left readable.

Tests

checkStorePresence is extracted as a pure function and table-tested, including that it does not create the directory as a side effect. Store permissions are asserted in internal/store.

go build ./..., go test ./... and go test -race ./... all pass.