remove q quit alias

`q` was introduced in round-5 as a peer Keyword variant alongside
`quit`. Per ADR-0023's "alias miss" critique, that was the wrong
shape — it surfaced `q` as a standalone command in completion
(only one of its kind), and required parallel parser + usage +
catalog + test entries. Drops the Keyword variant entirely; if
this ever needs to come back, it should arrive as an alias
annotation per ADR-0023, not as a peer keyword.

Tests still 769 passing.

src/dsl/parser.rs

@@ -469,8 +469,7 @@ fn command_parser<'a>()
     // BEFORE chumsky runs; the bare-keyword forms below
     // surface the `Path: None` / no-source variants for
     // empty-prompt completion + usage rendering.
-    let quit_cmd = choice((kw(Keyword::Quit), kw(Keyword::Q)))
-        .map(|()| Command::App(AppCommand::Quit));
+    let quit_cmd = kw(Keyword::Quit).map(|()| Command::App(AppCommand::Quit));
     let help_cmd = kw(Keyword::Help).map(|()| Command::App(AppCommand::Help));
     let rebuild_cmd =
         kw(Keyword::Rebuild).map(|()| Command::App(AppCommand::Rebuild));