docs(ops): link ops-monitor design from todo and README

Parent

Epic: #99
Design: doc/ops-monitor.md (Issue F)

Land / polish operator docs for the ops monitor:

Ensure doc/ops-monitor.md is in the tree
Point todo.md § Operations and monitoring at that design
README doc-index entry
Short operator cookbook: stuck device → hordectl activesync top → show → per-device log → admin action

todo.md no longer only has the one-line sketch; links to ops-monitor.md
README lists the doc
Cookbook covers the v1 CLI + admin path once those land (can ship doc skeleton earlier)

Can start anytime; finalize cookbook after CLI/admin issues land.

Cookbook path updated: use standalone horde-activesync top|show|summary (horde/base), not hordectl activesync. See epic #99

Documentation updates are ready in #108. This links the monitor from the README and TODO; the full operator design and cookbook ship with #107.