Keep the resource-monitor sparklines on a fixed 10-minute window

[ClaydeCode]

Summary

• Max reported that right after deploying core v35, the CPU sparkline stretched 2-3
  early samples across the full chart width instead of showing a short line near the
  right edge.
• Root cause: js/metrics.js's push() grows the history array from length 1 toward
  HISTORY_LENGTH (120), and fs-sparkline spaces its points evenly across the full
  width based on however many values it's given — so a freshly-started shard's 2-3
  samples got stretched edge to edge instead of representing their real ~15s span out
  of a 10-minute window.
• Fix: push() now left-pads the history to a constant HISTORY_LENGTH from the very
  first sample, with null for not-yet-collected slots. Per-sample spacing is then
  always fixed, so the line starts short at the right edge and grows leftward as real
  samples fill the window, then behaves as a normal sliding window once full.
  fs-sparkline itself needed no change — it already skips null points when drawing.

Test plan

• New unit tests in tests/unit/metrics.test.js cover: first-sample padding,
  right-anchored growth before the window fills, and sliding once full.
• Full unit suite: node --test tests/unit/ — 86/86 pass.