Status bar: configurable threshold and color for weekly usage warning

Feature Request: Two-stage configurable usage warning in status bar

The status bar currently shows the weekly usage warning in yellow starting at a relatively low threshold (~75%?). This appears on every session start, which becomes noisy at non-critical levels.

Proposed: Two-stage warning system

| Stage | Threshold | Color | Meaning |
|-------|-----------|-------|---------|
| 1 | ≥ 95% | dim / gray | "Getting there, heads up" |
| 2 | ≥ 98% | yellow / dark orange (current) | "Limit is genuinely close" |
| — | < 95% | hidden | No noise when there's plenty of headroom |

Suggested settings.json configuration

{
  "usageWarning": {
    "stages": [
      { "threshold": 95, "color": "dim" },
      { "threshold": 98, "color": "yellow" }
    ]
  }
}

Why two stages?

• Below 95%: No warning at all — pure noise, not actionable
• 95–97%: Show something subtle (dim/gray) — "be aware, plan ahead"
• 98%+: Current yellow is appropriate — limit is genuinely close

Seeing yellow at 78% creates false urgency. The current single-threshold approach has no middle ground between "silence" and "alarm".

Current behavior

Warning appears in yellow at ~75%+ on every session start — same alarming color regardless of whether you're at 76% or 99%.