Mark issues #02 (backtick lexing/parsing), #03 (command execution runtime), and #04 (argv parsing) as resolved. Add new issues for configuration (#08), programmable prompt (#09), and interactive command execution (#10).
Defines syntax decisions and implementation plan across 7 issues: - backtick command execution returning {code, stdout, stderr} - ${expr} interpolation in backticks - $SIGIL env var read/write - argv parsing, piping, and redirection (future)
