Add framework + monorepo detection CI/eval step

Problem

We're starting to rely on agentic detection of project structure (frameworks, monorepo layouts) in the wizard, but we don't yet have an automated way to catch regressions in that detection. The agentic source-map detection landed in #689 and the broader monorepo support is tracked in #113 — both would benefit from a guardrail.

Proposal

Add a CI/eval step that exercises framework and monorepo detection against a set of representative fixture projects, so we can verify detection behavior on every change and catch regressions before they ship.

Rough shape:

• A corpus of fixture repos / manifests covering common cases (single-app, pnpm/yarn/npm workspaces, mixed framework monorepos, Next.js in a subpackage, etc.)
• Run the detector against each fixture and assert on the structured JSON output (detected projects, frameworks, chosen manifest globs)
• Wire it into CI so detection changes are evaluated automatically

References

• #113 — Add support for monorepos
• #689 — feat: agentic detection for source-map upload flow

Filed from a Slack thread — reminder from the error tracking team to add a framework + monorepo detection CI/eval step.

Start by tracing the agentic detection work from #689 and the monorepo support in #113, then locate the repository's CI and evaluation entry points. Add representative fixture projects and assertions for structured detection output, and consider the work done when the detector runs against them automatically in CI.