Resolve username from host/container dir in activate-home.nix (flakes-7aac)

Replace flat knownHosts list with separate hosts/containers lists so activate-home.nix determines username (yjpark vs yj) from which dir the hostname is found in, rather than using $(id -un). Also update home-configs.nix fallback key to use ‘username@’ format.

Summary of Changes

• : replaced flat knownHosts with separate hosts and containers lists; shell script now checks hosts first (→ yjpark@$_host), then containers (→ yj@$_host), falling back to $(id -un)@ only when hostname is unknown
• home-configs.nix: updated fallback key from ${username} to "${username}@" to match new shell script format