userdirs: single resolver for the per-user config and cache dirs

Per review discussion on #1411: config-dir resolution was implemented four times (contexts, discovery, versioncheck, tokenstore), and "contexts" was the wrong home for a concept shared by version checks, discovery caches, and token storage.

internal/entireclient/userdirs is now the only place that resolves ~/.config/entire (Config: $ENTIRE_CONFIG_DIR, test fallback, home) and ~/.cache/entire (Cache: $XDG_CACHE_HOME/entire, test fallback, home). contexts.DefaultConfigDir and discovery.DefaultCacheDir are gone; all callers use userdirs directly.

Behavior fix folded in: the file token store's default path (ENTIRE_TOKEN_STORE=file without ENTIRE_TOKEN_STORE_PATH) previously hard-coded ~/.config/entire/tokens.json, ignoring ENTIRE_CONFIG_DIR; it now derives from userdirs.Config().

tokenstore's unexported store interface joins the ireturn allow list: the local lint --fix pass (nolintlint) strips the inline directives that CI's non-fix run requires, so the inline form flaps; the config entry satisfies both.