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

.bashrc.d/10_aliases.sh

@@ -17,8 +17,10 @@ alias sl='screen -list'
 alias sr='screen -r'
 
 # Process inspection
-alias psg='pstree | grep'
-alias ps-m='ps aux --sort -rss'
+# pstree may not be installed on all systems
+command -v pstree >/dev/null 2>&1 && alias psg='pstree | grep'
+# --sort is a GNU ps flag (Linux only)
+[[ "$(uname -s)" == "Linux" ]] && alias ps-m='ps aux --sort -rss'
 
 # Docker — short forms
 alias d='docker'
@@ -29,5 +31,10 @@ alias db='docker build'
 
 # Git helpers
 alias giturl='git config --get remote.origin.url'
-alias gitresetpw='git config --global credential.helper osxkeychain'
+# Git credential helper is OS-specific
+if [[ "$(uname -s)" == "Darwin" ]]; then
+    alias gitresetpw='git config --global credential.helper osxkeychain'
+else
+    alias gitresetpw='git config --global credential.helper store'
+fi
 alias git-clean='git-prune-branches'