feat(mem0-plugin): add PostHog telemetry across all hooks

Wire fire-and-forget PostHog telemetry into every lifecycle hook
(session_start, stop, pre_compact, task_completed, user_prompt,
post_tool_use) matching the same project key, endpoint, and payload
structure used by the Python SDK, CLI, and OpenClaw.

- 10% sampling on ALL events to keep PostHog costs predictable
- Anonymous distinct_id: MD5(api_key) or SHA-256(username)
- Opt-out via MEM0_TELEMETRY=false
- Platform detection: Claude Code / Cursor / Codex
- Never sends user content, API keys, or raw identifiers
- 16 new tests (110 total passing)

mem0-plugin/scripts/on_stop_codex.sh

@@ -33,6 +33,10 @@ if [ "$STOP_HOOK_ACTIVE" = "true" ]; then
   exit 0
 fi
 
+# Telemetry: fire before report() deletes stats file
+_TELEM_CAT=$(python3 "$SCRIPT_DIR/session_stats.py" peek 2>/dev/null | python3 -c "import json,sys; d=json.load(sys.stdin); print(len(d.get('categories',[])))" 2>/dev/null || echo "0")
+python3 "$SCRIPT_DIR/telemetry.py" stop --categories_count="$_TELEM_CAT" 2>/dev/null &
+
 # Session-end report (best-effort, must not break JSON output)
 REPORT=$(python3 "$SCRIPT_DIR/session_stats.py" report 2>/dev/null || echo "")
 REPORT_BLOCK=""