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/completion.rs

@@ -640,7 +640,7 @@ mod tests {
         // App-lifecycle commands now appear alongside DSL
         // commands in the entry-keyword set.
         for expected in &[
-            "quit", "q", "help", "rebuild", "save", "new", "load", "export",
+            "quit", "help", "rebuild", "save", "new", "load", "export",
             "import", "mode", "messages",
         ] {
             assert!(