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_task_completed.sh

@@ -37,4 +37,7 @@ Only store genuinely useful learnings — skip if the task was trivial.
 Include \`app_id\` = \`"$_PROJECT"\` as a top-level parameter in every \`add_memory\` call (not in metadata).
 EOF
 
+# Telemetry (background, fire-and-forget)
+python3 "$SCRIPT_DIR/telemetry.py" task_completed 2>/dev/null &
+
 exit 0