feat: Add pmg setup edit command (#235)

* feat: Add pmg setup edit command

* fix: Code review fixes

internal/shellwords/shellwords.go

@@ -0,0 +1,104 @@
+// Package shellwords provides POSIX-like splitting of shell command strings
+// into argv tokens. It is intentionally minimal: it handles the cases that
+// arise from environment variables like $VISUAL or $EDITOR, where users may
+// quote paths with spaces or pass flags. It is not a full shell parser — it
+// does not perform variable expansion, command substitution, or globbing.
+package shellwords
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Split splits s into tokens using POSIX-like rules:
+//
+//   - Whitespace (space, tab, newline) separates tokens.
+//   - Single quotes preserve their contents verbatim; no escapes are honored.
+//   - Double quotes preserve whitespace; backslash escapes only ", \, $, `.
+//   - Outside quotes a backslash escapes the next character literally.
+//   - Empty quoted strings ("" or '') produce an empty token.
+//
+// Split returns an error for unterminated quoted strings or a trailing
+// backslash with nothing to escape.
+func Split(s string) ([]string, error) {
+	var (
+		out      []string
+		buf      strings.Builder
+		inSingle bool
+		inDouble bool
+		escaped  bool
+		hasToken bool
+	)
+
+	flush := func() {
+		if hasToken {
+			out = append(out, buf.String())
+			buf.Reset()
+			hasToken = false
+		}
+	}
+
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		switch {
+		case escaped:
+			buf.WriteByte(c)
+			hasToken = true
+			escaped = false
+		case inSingle:
+			if c == '\'' {
+				inSingle = false
+			} else {
+				buf.WriteByte(c)
+			}
+			hasToken = true
+		case inDouble:
+			switch c {
+			case '"':
+				inDouble = false
+			case '\\':
+				if i+1 >= len(s) {
+					return nil, fmt.Errorf("dangling backslash")
+				}
+				next := s[i+1]
+				// Inside double quotes, backslash only escapes a few
+				// characters; otherwise it is literal.
+				if next == '"' || next == '\\' || next == '$' || next == '`' {
+					buf.WriteByte(next)
+					i++
+				} else {
+					buf.WriteByte(c)
+				}
+			default:
+				buf.WriteByte(c)
+			}
+			hasToken = true
+		default:
+			switch c {
+			case '\'':
+				inSingle = true
+				hasToken = true
+			case '"':
+				inDouble = true
+				hasToken = true
+			case '\\':
+				escaped = true
+			case ' ', '\t', '\n':
+				flush()
+			default:
+				buf.WriteByte(c)
+				hasToken = true
+			}
+		}
+	}
+
+	if inSingle || inDouble {
+		return nil, fmt.Errorf("unterminated quoted string")
+	}
+	if escaped {
+		return nil, fmt.Errorf("dangling backslash")
+	}
+	flush()
+
+	return out, nil
+}