fix(reporting): resolve SkillReport.model from the actual API response

[CalebKAston]

Summary

SkillReport.model only ever echoes the caller-configured model (options.model / trigger.model). When no model override is configured at any of the 4 levels a run can set one (per-skill, defaults.agent, defaults, per-trigger), this field is always undefined — even though the runtime already knows exactly which model answered each hunk.

The runtime normalizes this today as SkillRunResult.responseModel (sourced from the live API response, see runtimes/claude.ts's normalizeResult), but it only ever fed an opt-in HunkTrace object (captureTraces) and OTel spans — never the exported SkillReport.model that findings-file consumers actually read.

This threads responseModel from every analyzeHunk return path that has a live API response — including the circuit-breaker-opens-on-an-SDK-error path via hunkFailureFromCircuit, which now accepts and forwards it — up through analyzeFile and both independent report-construction sites: sdk/analyze.ts's runSkillAnalysis and the CLI's separate tasks.ts pipeline (tasks.ts drives analyzeFile directly with its own aggregation rather than calling runSkillAnalysis, so both need the same treatment).

Collapses to a single value via a new resolveResponseModel(models, fallback) helper in usage.ts when every hunk in the run agrees, falling back to the configured override when hunks disagree or none reported one — mirroring the existing model/models singular-vs-plural collapse pattern already used for auxiliaryUsageAttribution.

No schema change: SkillReportSchema.model already accepted a string, this only changes which value populates it.

Test plan

• pnpm lint && pnpm build && pnpm test — all green (93 test files, 1832 passing, 4 pre-existing skips)
• Regression coverage for: the single-model case, the fallback-on-disagreement case, the circuit-breaker-with-a-live-response case, and — since this value is computed per independent skill execution — that concurrent skills each get their own model correctly attributed rather than any cross-skill bleed