[experiment] python re-implement

SHELL_FEATURES.md

@@ -25,6 +25,7 @@ Blocked features are rejected before execution with exit code 2.
 - ✅ `ping [-c N] [-W DURATION] [-i DURATION] [-q] [-4|-6] [-h] HOST` — send ICMP echo requests to a network host and report round-trip statistics; `-f` (flood), `-b` (broadcast), `-s` (packet size), `-I` (interface), `-p` (pattern), and `-R` (record route) are blocked; count/wait/interval are clamped to safe ranges with a warning; multicast, unspecified (`0.0.0.0`/`::`), and broadcast addresses (IPv4 last-octet `.255`) are rejected — note: directed broadcasts on non-standard subnets (e.g. `.127` on a `/25`) are not blocked without subnet-mask knowledge
 - ✅ `ps [-e|-A] [-f] [-p PIDLIST]` — report process status; default shows current-session processes; `-e`/`-A` shows all; `-f` adds UID/PPID/STIME columns; `-p` selects by PID list
 - ✅ `printf FORMAT [ARGUMENT]...` — format and print data to stdout; supports `%s`, `%b`, `%c`, `%d`, `%i`, `%o`, `%u`, `%x`, `%X`, `%e`, `%E`, `%f`, `%F`, `%g`, `%G`, `%%`; format reuse for excess arguments; `%n` rejected (security risk); `-v` rejected
+- ✅ `python [-c CODE] [-h] [SCRIPT | -] [ARG ...]` — execute Python 3 source code using the gpython pure-Go interpreter (no CPython required); `-c CODE` runs inline code; a positional `SCRIPT` argument runs a file (via AllowedPaths sandbox); `-` reads from stdin; security sandbox removes all OS exec/write/spawn functions, replaces `open()` with a read-only AllowedPaths-aware version, and blocks `tempfile`/`glob` modules; source files and stdin are capped at 1 MiB; stdlib includes `math`, `string`, `sys`, `os` (read-only), `binascii`, `json` (dumps only; loads raises ValueError), `collections` (OrderedDict, Counter, deque, defaultdict — defaultdict default_factory raises NotImplementedError if provided); blocked modules: `subprocess`, `socket`, `ctypes`, `tempfile`, `glob`, `threading`, `multiprocessing`, `asyncio`, `re` (raises ImportError on import); known limitation: `import os.path` raises ImportError — use `from os import path` instead
 - ✅ `sed [-n] [-e SCRIPT] [-E|-r] [SCRIPT] [FILE]...` — stream editor for filtering and transforming text; uses RE2 regex engine; `-i`/`-f` rejected; `e`/`w`/`W`/`r`/`R` commands blocked
 - ✅ `strings [-a] [-n MIN] [-t o|d|x] [-o] [-f] [-s SEP] [FILE]...` — print printable character sequences in files (default min length 4); offsets via `-t`/`-o`; filename prefix via `-f`; custom separator via `-s`
 - ✅ `tail [-n N|-c N] [-q|-v] [-z] [FILE]...` — output the last part of files (default: last 10 lines); supports `+N` offset mode; `-f`/`--follow` is rejected