feat(sandbox): pricing benchmark tracking provider rates over time

[HeyGarrison]

Summary

Adds a sandbox-pricing benchmark that turns provider pricing into a tracked time series, same shape as sandbox-dax: weekly run → results/sandbox-pricing/<YYYY-MM-DD>.json + latest.json, which dotcom reads off raw.githubusercontent.com (used to keep /compare honest over time).

• benchmarks/sandbox/pricing.ts — PRICING_PROVIDERS: 26 sandbox providers, each with its public pricing page URL and extractors ({key, pattern, scale?}). Each pattern captures the literal price figure printed on the page (e.g. 0.000014 → ×3600 for per-second → per-hour). hourlyRate(rates) reduces extracted units to a comparable $/hr at the Dax spec (8 vCPU / 16 GiB); providers quoted per-core (Beam, Modal) use 4 * coreHour + 16 * gibHour.
• benchmarks/sandbox/pricing.bench.ts — declarative bench, 1 iteration per provider, all fetched in parallel, no credentials needed. Task returns data: {hourlyRate, rates, fieldsMatched, fieldsTotal, httpStatus}; extraction misses and HTTP errors are recorded rather than thrown — a provider whose page stops matching is the signal this benchmark exists to catch.
• pricing-legacy-results.ts — onComplete bridge writing the dated + latest.json results files.
• .github/workflows/sandbox-pricing-benchmarks.yml — weekly Wednesday run + workflow_dispatch, commits results to master like the Dax workflow.

Verified locally with --no-ingest: 24/26 providers extract cleanly and match the verified rates on the compare page (e.g. e2b $0.6624, modal $0.9518, vercel $1.3632). Two known misses that stay recorded as misses: CodeSandbox (403 bot-block) and CreateOS (JS-rendered page).

Link to Devin session: https://app.devin.ai/sessions/6f63e57c31da447b95f7580d3324b40b
Open in Devin Desktop: https://app.devin.ai/desktop/session/6f63e57c31da447b95f7580d3324b40b?variant=devin
Requested by: @HeyGarrison

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