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/_identity.sh

@@ -83,5 +83,12 @@ else
 fi
 export MEM0_AUTO_SAVE MEM0_AUTO_SEARCH MEM0_SEARCH_LIMIT MEM0_RETENTION_SESSION_DAYS MEM0_CONFIDENCE_THRESHOLD MEM0_GLOBAL_SEARCH MEM0_DEBUG
 
+# Resolve base URL: MEM0_BASE_URL env var > settings.json "base_url" > hosted platform
+if [ -z "${MEM0_BASE_URL:-}" ] && command -v python3 >/dev/null 2>&1; then
+  MEM0_BASE_URL=$(PYTHONPATH="$_SCRIPT_DIR" python3 -c "from _identity import resolve_base_url; print(resolve_base_url())" 2>/dev/null || echo "https://api.mem0.ai")
+fi
+: "${MEM0_BASE_URL:=https://api.mem0.ai}"
+export MEM0_BASE_URL
+
 # Also resolve project context
 . "$_SCRIPT_DIR/_project.sh"