Implement uniq builtin command

README.md

@@ -66,7 +66,7 @@ Linux, macOS, and Windows.
 
 ```
 tests/scenarios/
-├── cmd/          # builtin command tests (echo, cat, head, tail, wc, ...)
+├── cmd/          # builtin command tests (echo, cat, head, tail, uniq, wc, ...)
 └── shell/        # shell feature tests (pipes, variables, control flow, ...)
 ```
 

SHELL_FEATURES.md

@@ -14,6 +14,7 @@ Blocked features are rejected before execution with exit code 2.
 - ✅ `head [-n N|-c N] [-q|-v] [-z] [FILE]...` — output the first part of files (default: first 10 lines)
 - ✅ `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
 - ✅ `true` — return exit code 0
+- ✅ `uniq [OPTION]... [INPUT]` — report or omit repeated lines
 - ✅ `wc [-l] [-w] [-c] [-m] [FILE]...` — count lines, words, bytes, or characters in files
 - ❌ All other commands — return exit code 127 with `<cmd>: not found` unless an ExecHandler is configured