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_session_start.sh

@@ -162,4 +162,7 @@ is being captured to mem0 in the background as `metadata.type=compact_summary`.
 EOF
 fi
 
+# Telemetry (background, fire-and-forget)
+python3 "$SCRIPT_DIR/telemetry.py" session_start --source="$SOURCE" --memory_count="${MEM0_COUNT:-0}" 2>/dev/null &
+
 exit 0