feat(mem0-plugin): support self-hosted Mem0 server via MEM0_BASE_URL

Adds an optional MEM0_BASE_URL override (env var, falling back to
settings.json's base_url, defaulting to the hosted Platform unchanged)
so the plugin's REST-backed hooks can talk to a self-hosted server/
deployment instead of api.mem0.ai.

A new scripts/_api.py centralizes the hosted-vs-self-hosted protocol
differences (unversioned paths, X-API-Key auth, agent_id instead of
app_id) that every hook previously hardcoded, and all 9 urllib call
sites now route through it.

MCP tools have no self-hosted backend (server/ implements no MCP), so
they continue to talk to the hosted Platform regardless of
MEM0_BASE_URL; this is documented rather than silently left unclear.
Coding-category customization also has no self-hosted SDK equivalent
and now skips cleanly with a log message instead of failing.

Closes MEM-5647

integrations/mem0-plugin/scripts/on_session_start.sh

@@ -71,37 +71,15 @@ fi
 
 MEM0_COUNT="?"
 if command -v python3 >/dev/null 2>&1; then
-  MEM0_COUNT=$(python3 -c "
-import json, os, urllib.request, urllib.error
+  MEM0_COUNT=$(PYTHONPATH="$SCRIPT_DIR" python3 -c "
+import os
+from _api import count_memories
+
 api_key = os.environ.get('MEM0_API_KEY', '')
 user_id = os.environ.get('MEM0_RESOLVED_USER_ID', 'default')
-app_id = os.environ.get('MEM0_PROJECT_ID', '')
+project_id = os.environ.get('MEM0_PROJECT_ID', '')
 global_search = os.environ.get('MEM0_GLOBAL_SEARCH', 'false') == 'true'
-
-def get_count(filters):
-    body = json.dumps({'filters': filters}).encode()
-    req = urllib.request.Request(
-        'https://api.mem0.ai/v3/memories/?page=1&page_size=1',
-        headers={'Authorization': f'Token {api_key}', 'Content-Type': 'application/json'},
-        data=body, method='POST',
-    )
-    with urllib.request.urlopen(req, timeout=5) as r:
-        data = json.loads(r.read())
-        if isinstance(data, dict) and 'count' in data:
-            return data['count']
-        if isinstance(data, list):
-            return len(data)
-    return 0
-
-try:
-    if global_search:
-        filters = {'OR': [{'user_id': '*'}]}
-    else:
-        filters = {'AND': [{'user_id': user_id}, {'app_id': app_id}]}
-    total = get_count(filters)
-    print(total)
-except Exception:
-    print('?')
+print(count_memories(api_key, user_id, project_id, global_search=global_search))
 " 2>/dev/null || echo "?")
 fi