Add shell aliases via lush.aliases table (issue #28)

Aliases rewrite the command string before pipeline splitting, so alias
values may contain pipes (e.g. lush.aliases.foo = "echo hi |"). Expansion
happens once per dispatch to prevent recursion.

testes/lush/lushlib.lua

@@ -93,6 +93,87 @@ do
 end
 
 
+-- === lush.aliases exists and is a table ===
+
+do
+  assert(type(lush.aliases) == "table", "lush.aliases missing")
+  -- iterable (pairs works on it)
+  local count = 0
+  for k, v in pairs(lush.aliases) do count = count + 1 end
+  assert(count == 0, "aliases should start empty")
+end
+
+
+-- === simple alias ===
+
+do
+  lush.aliases.myecho = "echo hello"
+  local r = `myecho`
+  assert(r.code == 0, "alias failed")
+  assert(r.stdout == "hello\n", "expected 'hello\\n', got: " .. r.stdout)
+  lush.aliases.myecho = nil
+end
+
+
+-- === alias with extra args ===
+
+do
+  lush.aliases.myecho = "echo hello"
+  local r = `myecho world`
+  assert(r.code == 0, "alias with args failed")
+  assert(r.stdout == "hello world\n",
+         "expected 'hello world\\n', got: " .. r.stdout)
+  lush.aliases.myecho = nil
+end
+
+
+-- === alias does not recurse ===
+
+do
+  lush.aliases.echo = "echo aliased"
+  local r = `echo extra`
+  assert(r.code == 0, "aliased echo failed")
+  assert(r.stdout == "aliased extra\n",
+         "expected 'aliased extra\\n', got: " .. r.stdout)
+  lush.aliases.echo = nil
+end
+
+
+-- === alias removed ===
+
+do
+  lush.aliases.myecho = "echo hello"
+  lush.aliases.myecho = nil
+  -- myecho should now be a normal (nonexistent) command
+  local r = `myecho`
+  assert(r.code ~= 0, "removed alias should not work")
+end
+
+
+-- === alias with pipe in value ===
+
+do
+  lush.aliases.grephello = "echo hello | grep"
+  local r = `grephello hello`
+  assert(r.code == 0, "alias with pipe failed")
+  assert(r.stdout == "hello\n",
+         "expected 'hello\\n', got: " .. r.stdout)
+  lush.aliases.grephello = nil
+end
+
+
+-- === alias works with lush.command ===
+
+do
+  lush.aliases.myecho = "echo via-command"
+  local r = lush.command("myecho")
+  assert(r.code == 0)
+  assert(r.stdout == "via-command\n",
+         "expected 'via-command\\n', got: " .. r.stdout)
+  lush.aliases.myecho = nil
+end
+
+
 -- === OP_LUSH still works (backtick, $VAR, !cmd syntax) ===
 
 do