Implement sed builtin command

SHELL_FEATURES.md

@@ -16,6 +16,7 @@ Blocked features are rejected before execution with exit code 2.
 - ✅ `head [-n N|-c N] [-q|-v] [FILE]...` — output the first part of files (default: first 10 lines); `-z`/`--zero-terminated` and `--follow` are rejected
 - ✅ `ls [-1aAdFhlpRrSt] [FILE]...` — list directory contents
 - ✅ `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
+- ✅ `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
 - ✅ `tr [-cdsCt] SET1 [SET2]` — translate, squeeze, and/or delete characters from stdin