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_pre_compact.sh

@@ -21,6 +21,7 @@ INPUT=$(cat)
 # Fire REST API transcript backup in background (safety net if agent
 # can't complete the add_memory call before compaction finishes)
 echo "$INPUT" | python3 "$SCRIPT_DIR/on_pre_compact.py" --source=pre-compaction 2>/dev/null &
+python3 "$SCRIPT_DIR/telemetry.py" pre_compact 2>/dev/null &
 
 cat <<'EOF'
 ## CRITICAL: Pre-Compaction Session Summary