docs: document COMMAND-mode Tab completion

Add the Tab completion keybinding to README, QUICKREF, and the tnt.1 man
page in the same change set as the feature.

Author: m1ngsama

README.md

@@ -126,6 +126,8 @@ Ctrl+C     - Exit chat
 :help                - Show concise manual
 :clear               - Clear command output
 :q, :quit, :exit     - Disconnect
+Tab                  - Complete command name; ambiguous prefixes list candidates
+                       (also completes :theme/:lang values and :msg usernames)
 Up/Down              - Browse command history
 ESC                  - Return to NORMAL mode
 ```

docs/QUICKREF.md

@@ -42,6 +42,7 @@ COMMANDS (COMMAND mode, prefix with :)
   lang [en|zh]           show or switch UI language
   clear                  clear output
   q / quit / exit        disconnect
+  Tab                    complete command / argument (lists candidates if ambiguous)
 
 INSERT MODE
   /me <action>           action message

tnt.1

@@ -234,6 +234,7 @@ l l.
 :help	Show concise manual
 :clear	Clear command output
 :q, :quit, :exit	Disconnect
+Tab	Complete the command name or first argument; ambiguous prefixes list candidates
 Up/Down	Browse command history
 ESC	Cancel and return to NORMAL
 .TE