feat(usagereport): utilization-aware status message framing (closes #41)

[Defilan]

What

Rewrites the UsageReport Ready-condition message (and adds a matching section to infercost status) so the headline cost is never shown without utilization context.

Why

Fixes #41

Period 2026-04-23: $0.0660 across 3,993 tokens is correct but misleading — readers divide and conclude "$16/MTok, worse than every cloud." The launch runbook says InferCost's differentiator is honest framing, including when cloud is cheaper; the bare string doesn't live up to that. The new framing puts the amortized number next to the utilization that explains it, plus the marginal rate and the break-even comparison:

Period 2026-04-23: $0.0660 across 3,993 tokens.
Amortized: $16.52/MTok at 4% utilization.
Marginal: $0.0040/MTok.
Break-even with Anthropic/claude-opus-4-6: 142K tokens/day (you served 6K).

How

• internal/report.StatusMessage(UsageReportStatus) — a pure, table-tested formatter, shared by the controller and CLI so their framing is identical. Marginal line is omitted when 0 (no energy sampled → "$0.0000" would be misleading); break-even line omitted when there are no targets.
• Controller — applyStatusIfChanged sets the Ready message via the formatter.
• CLI — infercost status gains a "USAGE REPORTS" section printing the same framing per report.
• Tests — formatter table tests (humanize, commas, conditional lines) + a controller assertion that the message carries the new framing.
• Grafana — the utilization %, $/MTok panels and the new break-even gauge (Break-even tokens-per-day calculator per cloud provider #39) already carry this visually; a Prometheus dashboard can't render a CRD condition string, so no message panel is added.

Note on stacking

Builds on #39 / PR #67 (the break-even line reads status.breakEvenAnalysis). This PR is based on feat/break-even-calculator; merge #67 first (GitHub will retarget this to main).

Checklist

• Status condition message updated in applyStatusIfChanged
• CLI infercost status output matches
• Unit tests assert the new phrasing
• make test + make lint pass locally
• Signed off (DCO); conventional commit
• Grafana: covered by existing panels + break-even gauge (documented above)