refactor(docs): slim README and improve completion script UX

- Reduce README from 530+ lines to ~110 lines, focusing on CLI core
- Separate ecosystem extensions (cc-switch, Metabot, Skills) into
  an optional section
- Add unsupported shell hint in install_completion.sh

scripts/install_completion.sh

@@ -38,4 +38,9 @@ if [[ "$USER_SHELL" == "zsh" ]]; then
     install_zsh_completion
 elif [[ "$USER_SHELL" == "bash" ]]; then
     install_bash_completion
+else
+    echo "Shell '$USER_SHELL' is not supported for automatic completion installation."
+    echo "To install completions manually, run one of the following commands:"
+    echo "  zsh:  _MYCLAUDE_COMPLETE=zsh_source $VENV_MYCLAUDE > /path/to/completions/_myclaude"
+    echo "  bash: _MYCLAUDE_COMPLETE=bash_source $VENV_MYCLAUDE > /path/to/completions/myclaude.bash"
 fi