docs: add literate prose to naming-drift aliases and HITL gateway changes

Adds prose sections before every code block to satisfy the
prose-before-code discipline. Each backward-compatibility alias
(process-signal, perceive-gate, reason-gate, act-gate, inject-stimulus)
now has its own subsection explaining why it exists and what new code
should use instead.

Also:
- Fixes double #+end_src in core-loop-perceive.org
- Renames inject-stimulus → stimulus-inject in heartbeat-start and
  client-handle-connection (both already had aliases)
- Adds HITL interception prose to gateway-manager.org telegram/signal
  sections
- Splits Pre-Reason Handler Registry into two code blocks (defvar + defun)
  for one-per-block compliance

lisp/core-communication.lisp

@@ -71,7 +71,7 @@
                                                      nil))))
                    (format stream "~a" (frame-message health-msg))
                    (finish-output stream)))
-                (t (inject-stimulus msg :stream stream))))))
+                (t (stimulus-inject msg :stream stream))))))
       (error (c) (log-message "CLIENT ERROR: ~a" c)))
     (ignore-errors (usocket:socket-close socket))))