Auto-adjust cost histogram bucket intervals (#436)

[joelteply]

Summary

• Server-side autoInterval() picks optimal bucket width based on time range: 1h→5m, 6h→15m, 24h→1h, 72h→3h, 7d→6h, 30d→1d
• Targets 12-24 buckets for visual clarity at any zoom level
• Removed duplicated interval maps from ContinuumMetricsWidget and MetricsDetailWidget — single source of truth in AICostServerCommand
• Callers can still override with explicit interval param

Closes #436.

Test plan

• TypeScript compilation passes
• Visual verification: cost dashboard shows appropriate bucket granularity at each time range

[Copilot]

Pull request overview

This PR centralizes time-series histogram bucket sizing for the cost dashboard by moving “adaptive interval” selection to the ai/cost server command, allowing widgets to omit client-side interval maps while preserving the ability to override with an explicit interval.

Changes:

• Add server-side autoInterval() selection and use it when params.interval is not provided.
• Remove duplicated client-side intervalMap logic from the two metrics widgets.
• Keep support for explicit interval overrides via the existing command parameter.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/widgets/metrics-detail/MetricsDetailWidget.ts	Removes client-side interval mapping so the server chooses bucket width automatically.
src/widgets/continuum-metrics/ContinuumMetricsWidget.ts	Removes duplicated adaptive interval map and relies on server defaults.
src/commands/ai/cost/server/AICostServerCommand.ts	Introduces autoInterval(spanMs) and uses it to pick a default interval for time-series generation.

Comments suppressed due to low confidence (1)

src/commands/ai/cost/server/AICostServerCommand.ts:305

• parseIntervalToMs() accepts intervals like "0m"/"0h" (regex allows 0), which makes intervalMs = 0 and will cause an infinite loop in generateTimeSeries() (bucketStart never advances). Please validate that the parsed value is >= 1 (or that intervalMs > 0) and throw a clear error otherwise.

  private parseIntervalToMs(interval: string): number {
    const match = interval.match(/^(\d+)(m|h|d|w)$/);
    if (!match) {
      throw new Error(`Invalid interval format: ${interval}. Use "5m", "1h", "6h", "1d", "1w"`);
    }

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[Copilot]

The docstring claims this targets 12–24 buckets, but the current mapping can exceed that (e.g., 7d with 6h => 28 buckets; 30d with 1d => 30 buckets). Please update the comment to reflect the actual behavior/target range so future callers don’t rely on an incorrect guarantee.

Suggested change

	* Targets 12-24 buckets for visual clarity at any zoom level.
	* Aims for a reasonable number of buckets for visual clarity (typically
	* around a dozen to a few dozen), but may exceed 24 buckets for longer
	* ranges (e.g., ~28 buckets for 7d@6h, ~30 buckets for 30d@1d).