repl: :mcp tool — disambiguate "no alias" vs "unknown alias" errors

Surfaced by Phase 7 verify test case #29: typing :mcp tool list_dir (no
dot) printed "unknown alias: nil" instead of a useful diagnostic. The
parse failure was being conflated with the alias-not-found case.

Now:
  :mcp tool list_dir          -> tool name missing alias prefix: list_dir
  :mcp tool unknown_alias.x   -> unknown alias: unknown_alias
  :mcp tool known_alias.bogus -> unknown tool: known_alias.bogus

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

repl.lua

@@ -483,9 +483,13 @@ function M.run(config)
                     renderer.status("usage: :mcp tool <alias.name>"); return
                 end
                 local alias, tname = name:match("^([^.]+)%.(.+)$")
-                local sess = alias and mcp_sessions[alias]
+                if not alias then
+                    renderer.status("tool name missing alias prefix: " .. name)
+                    return
+                end
+                local sess = mcp_sessions[alias]
                 if not sess then
-                    renderer.status("unknown alias: " .. tostring(alias))
+                    renderer.status("unknown alias: " .. alias)
                     return
                 end
                 local found