Closes#4284. hyperhive.agent.claude_md.lines is already exported (see
hive-agent's claude_md_watch module) but had no dashboard panel — this
is the display half.
Same bargauge shape as the two sibling per-agent gauges already in this
file (container storage, active time): sum by (agent), instant query,
lastNotNull reduction, since it's a continuously-live gauge rather than
a cumulative counter. Metric name uses the dotted OTLP form per this
file's own documented convention for this store.
93ed6a977f