fix: Linux/Ubuntu compatibility for shell configs and dotfiles manager

00_env.sh:
  - yaml2json: use python3 (python3 first, python fallback) — Ubuntu 20+ has no 'python'
  - pyenv init: moved out of Darwin-only block; now runs everywhere if pyenv installed
  - xterm ls alias: split by OS — Linux gets 'ls --color=auto', macOS gets 'ls -G'

10_aliases.sh:
  - psg: guard with 'command -v pstree' (not installed by default on Ubuntu)
  - ps-m: Linux-only (--sort flag is GNU ps, not available on macOS)
  - gitresetpw: OS-conditional — osxkeychain on Darwin, store on Linux

dotfiles_manager.sh:
  - Replace both hardcoded 'brew install gnupg' die messages with _require_gpg()
    helper that gives apt-get hint on Linux and brew hint on macOS

scripts/dotfiles_manager.sh

@@ -129,6 +129,15 @@ _authed_url() {
     fi
 }
 
+# Check gpg is installed with OS-appropriate install hint
+_require_gpg() {
+    command -v gpg &>/dev/null && return 0
+    case "$(uname -s)" in
+        Darwin*) die "gpg not installed. Install with: brew install gnupg" ;;
+        *)       die "gpg not installed. Install with: sudo apt-get install gnupg" ;;
+    esac
+}
+
 # -----------------------------------------------------------------------
 # COMMAND: init
 # -----------------------------------------------------------------------
@@ -692,7 +701,7 @@ SSH_CONFIG
 # -----------------------------------------------------------------------
 cmd_ssh_export() {
     bold "=== Export SSH Private Keys (GPG-encrypted) ==="
-    command -v gpg &>/dev/null || die "gpg not installed. Install with: brew install gnupg"
+    _require_gpg
 
     local dotfiles_ssh="$DOTFILES_DIR/.ssh/keys"
     mkdir -p "$dotfiles_ssh"
@@ -752,7 +761,7 @@ cmd_ssh_export() {
 # -----------------------------------------------------------------------
 cmd_ssh_import() {
     bold "=== Import SSH Private Keys (GPG decrypt) ==="
-    command -v gpg &>/dev/null || die "gpg not installed. Install with: brew install gnupg"
+    _require_gpg
 
     local dotfiles_ssh="$DOTFILES_DIR/.ssh/keys"
     local gpg_keys=()