fix(limits): lower the session ceiling to 462 so the layout budget holds

An owner's tab layout holds at most MAX_TAB_REFS (512) refs, counting saved web
tabs (up to MAX_WEBVIEWS, 50 by default) as well as sessions. With a ceiling of
500, session creation could still fail with HTTP 500 on the layout limit before
the capacity message. 512 - 50 = 462; a test pins the ceiling plus MAX_WEBVIEWS
inside MAX_TAB_REFS so the two limits cannot drift apart again.

Co-Authored-By: Christian Haberl <116439789+christianhaberl@users.noreply.github.com>
Co-Authored-By: Claude <noreply@anthropic.com>

docs/wiki/HTTP-API.md

@@ -185,7 +185,7 @@ curl -s -X POST "$API/api/sessions/$ID/custom-model" -H 'Content-Type: applicati
 
 | Limit                 | Default                                    |
 | --------------------- | ------------------------------------------ |
-| Max sessions          | 50                                          |
+| Max sessions          | 50 (`CODEMAN_MAX_SESSIONS`)                 |
 | Max agent windows     | 500                                         |
 | Max SSE clients       | 100                                         |
 | Terminal buffer       | 32 MB per session                           |