Add nrepl-show-messages to pop up the nREPL messages buffer

Previously the only way to find the messages buffer was to remember
its template-generated name and dig it out of the buffer list.  The
new command finds live nrepl-messages-mode buffers; with one
connection it pops it directly, with several it prompts.  Bundles a
CHANGELOG entry for all three nREPL log UX changes on this branch.

Author: bbatsov

CHANGELOG.md

@@ -12,6 +12,7 @@
 - New `cider-repl-history-doctor` command: walks `cider-repl-input-history` looking for entries whose parens don't balance under Clojure syntax, shows each in a side buffer, and asks whether to delete it.  When done, rewrites `cider-repl-history-file` if one is configured.  Useful for cleaning up history after a typo got committed that breaks `cider-repl-history` rendering (see [#3915](https://github.com/clojure-emacs/cider/issues/3915)).
 - Recognize [let-go](https://github.com/nooga/let-go) (a Clojure dialect implemented in Go) as a known nREPL runtime.  `cider-runtime` returns `let-go` for these connections and the connection info line shows the runtime version, e.g. `CLJ project@localhost:2137 (let-go 1.0)`.
 - Decouple the nREPL transport layer from CIDER's UI layer (closes [#1099](https://github.com/clojure-emacs/cider/issues/1099)). `nrepl-make-eval-handler` is now CIDER-agnostic: it no longer references `nrepl-namespace-handler-function`, `nrepl-err-handler-function`, `nrepl-need-input-handler-function`, or any hardcoded UI strings. New `:on-ns` and `:on-status` keyword slots let any consumer wire up their own namespace tracking and status handling. The editor-level `cider-make-eval-handler` wraps it with CIDER's UI behavior (ns tracking, default error handler, need-input prompt, "Evaluation interrupted." / "Namespace not found." messages); in-tree callers all use it.
+- New nREPL message log commands: `nrepl-show-messages` pops up the messages buffer (prompting when there are multiple connections), and `c` in `nrepl-messages-mode` runs the new `nrepl-clear-messages` to wipe the buffer. Auto-follow on new entries now leaves windows alone if you've scrolled back to read history; only windows already at end-of-buffer get scrolled to the latest message.
 
 ### Bugs fixed
 

lisp/nrepl-client.el

@@ -1307,6 +1307,25 @@ This in effect enables or disables the logging of nREPL messages."
   (let ((inhibit-read-only t))
     (erase-buffer)))
 
+(defun nrepl-show-messages ()
+  "Pop up an nREPL messages buffer.
+With more than one active connection, prompt for which one to show.
+If `nrepl-log-messages' is nil no buffer exists yet, so signal an
+error pointing at `nrepl-toggle-message-logging'."
+  (interactive)
+  (let ((buffers (seq-filter (lambda (b)
+                               (with-current-buffer b
+                                 (derived-mode-p 'nrepl-messages-mode)))
+                             (buffer-list))))
+    (pcase (length buffers)
+      (0 (user-error "No nREPL messages buffer exists; enable logging with `nrepl-toggle-message-logging'"))
+      (1 (pop-to-buffer (car buffers)))
+      (_ (pop-to-buffer
+          (get-buffer
+           (completing-read "nREPL messages buffer: "
+                            (mapcar #'buffer-name buffers)
+                            nil t)))))))
+
 (defcustom nrepl-message-colors
   '("red" "brown" "coral" "orange" "green" "deep sky blue" "blue" "dark violet")
   "Colors used in the messages buffer."