[amandhaw]

Summary

Adds ListRegions — GET /v2/harness-runtime/regions, the per-team region catalog.

regions, _, err := client.HostedAgents.ListRegions(ctx)
// {"regions":[{"slug":"mem1","is_default":false},{"slug":"ric1","is_default":true}]}

slug, ok := regions.DefaultRegion() // "ric1", true

The default region is always listed and marked is_default; every other open region is listed only when the team holds its grant. The set is exactly what CreateSession and CreateSessionFromEnvironment placement admit, so a slug returned here is accepted and a slug missing here is refused — a picker built from the catalog never offers a region a create rejects.

This is the SDK support the CLI's agents regions command needs; that work follows separately.

Draft: do not merge yet

The endpoint is not in production. The implementing service change is merged but not yet deployed, and the route is still refused at the edge until the matching allow-list change lands. Opened early so CI runs and the CLI work has something to build against.

Changes

File
hosted_agent_regions.go	types, ListRegions, DefaultRegion()
hosted_agent_regions_test.go	tests
hosted_agents.go	path const and the interface method

Types live in a per-concern file with the interface method central, matching hosted_agent_checkpoints.go, hosted_agent_configs.go and the other siblings.

Notes

• Team-scoped, not host-scoped. The catalog is the same on every regional host, so one call enumerates the regions rather than one call per HostedAgentsRegionalBaseURL.
• DefaultRegion() exists because every caller rendering a picker needs "which one is preselected", and the alternative is each of them looping over the slice. It reports not-found rather than falling back to the first entry: guessing a default would place a session in a region the caller never chose.
• No CHANGELOG.md or version bump, per CONTRIBUTING.md step 3 — those are a release-time change in their own PR.

Testing

gofmt and go vet clean. Four new tests: the catalog, the default-only case a team with no grants sees, the error path, and a table for DefaultRegion() including the nil receiver. Full root package suite passes with no regressions from the interface change.