feat(make): extract completion install to standalone script

Move shell-specific completion generation from Makefile inline
logic to scripts/install_completion.sh for easier maintenance.

scripts/install_completion.sh

@@ -0,0 +1,41 @@
+#!/bin/bash
+# Install shell completions for myclaude to user directory only
+# Usage: install_completion.sh <venv_myclaude_path> <comp_dir>
+
+VENV_MYCLAUDE="$1"
+COMP_DIR="$2"
+
+if [[ -z "$VENV_MYCLAUDE" || -z "$COMP_DIR" ]]; then
+    echo "Usage: $0 <venv_myclaude_path> <comp_dir>" >&2
+    exit 1
+fi
+
+USER_SHELL=$(basename "$SHELL")
+
+install_zsh_completion() {
+    mkdir -p "$COMP_DIR" 2>/dev/null || { echo "Error: cannot create $COMP_DIR" >&2; return 1; }
+    if _MYCLAUDE_COMPLETE=zsh_source "$VENV_MYCLAUDE" > "$COMP_DIR/_myclaude" 2>/dev/null; then
+        echo "Installed zsh completion: $COMP_DIR/_myclaude"
+        return 0
+    else
+        echo "Warning: failed to generate zsh completion" >&2
+        return 1
+    fi
+}
+
+install_bash_completion() {
+    mkdir -p "$COMP_DIR" 2>/dev/null || { echo "Error: cannot create $COMP_DIR" >&2; return 1; }
+    if _MYCLAUDE_COMPLETE=bash_source "$VENV_MYCLAUDE" > "$COMP_DIR/myclaude.bash" 2>/dev/null; then
+        echo "Installed bash completion: $COMP_DIR/myclaude.bash"
+        return 0
+    else
+        echo "Warning: failed to generate bash completion" >&2
+        return 1
+    fi
+}
+
+if [[ "$USER_SHELL" == "zsh" ]]; then
+    install_zsh_completion
+elif [[ "$USER_SHELL" == "bash" ]]; then
+    install_bash_completion
+fi