docs: correct stale documentation and drop planning artifacts

The docs had drifted from the code and described things that never existed.

- architecture.md documented a QuitCommand class (quit/exit are aliases on
  MenuCommand) and a GameIO interface of read()/write(String); the real one is
  readLine() plus print(StyledText) with presentation hooks. Both corrected,
  along with the package diagram and the game-loop sketch.
- conventions.md listed a package tree missing save, menu, io.text and
  command.impl. Updated, including why MapLayout lives in game.
- data-structures.md claimed LinkedList was deliberately avoided (Pathfinder
  uses one for O(1) addFirst during path reconstruction), named the wrong queue
  for the Swing bridge, and listed an input history that was never built.
- Removed enhancement-ideas.md and implementation-status.md: a backlog and a
  phase tracker, neither of which documents the delivered project.

Also stop tracking the built jars. They still ship in the submission, but a
22 MB binary that changes on every build does not belong in git history.

.gitignore

@@ -12,6 +12,3 @@ Thumbs.db
 
 # Nextcloud sync
 .sync-exclude.lst
-
-# Build artifacts (kept on disk for the submission ZIP, not in git)
-release/*.jar