Fix Isolated Ladder NavMesh on Threadplate

[sunzenshen]

Fixes connections around an isolated ladder on Threadplate, to absence of which was causing bots to avoid that ladder.

[sunzenshen]

Why this PR

A ladder in Threadplate was only connected to the NavMesh on one side, which prevented bots from using that ladder to access a ghost spawn:

Manual fix steps

Created a new NavArea with nav_begin_area and nav_end_area, and connected the edges to the surrounding NavAreas:

Verification

After running nav_analyze, ordered some bots to climb the ladder with the following script setup:

sv_neo_bot_cmdr_enable 1;
sv_neo_bot_cmdr_debug_pause_uncommanded 1;
neo_ctg_round_timelimit 99;