Remove legacy registry format internals from toolhive

Summary

Phase 1 of removing the legacy registry format from toolhive: the parse-side internals and the registry_legacy_format error code. Phase 2 — aligning the registry API with the toolhive-registry-server v0.1 surface — is tracked separately in stacklok/toolhive#5279, which is what actually unblocks stacklok/toolhive-core#106.

The legacy ToolHive registry format is no longer produced anywhere after the 2026-05-13 cleanups in toolhive-catalog (#1215), toolhive-core (#105), and toolhive (#5273).

Scope

Parse-side internals:

• pkg/registry/legacyhint/
• pkg/registry/convert.go (ConvertJSON)
• pkg/registry/errors.go (LegacyFormatError)
• pkg/registry/upstream_parser.go (errLegacyFormat + sniffer fallback)
• pkg/registry/provider_remote.go (legacyhint.Looks() call)
• cmd/thv/app/registry_convert.go (thv registry convert CLI)

v1 API error code:

• pkg/api/v1/registry.go — registry_legacy_format error code and LegacyFormatError propagation
• pkg/api/v1/registry_test.go

Open questions

1. Keep legacyhint + the registry_legacy_format error code for one release as a friendly migration hint, or delete now?
2. Does thv registry convert stay as a one-shot migration tool, or go with the rest?

Acceptance criteria

• No remaining imports of legacy-aware parse types in toolhive (modulo a deprecation window if we keep one).
• No emission of registry_legacy_format from any toolhive code path.
• task test, task test-e2e, task docs all green/regenerated.

References

• Phase 2: stacklok/toolhive#5279
• Landed: stacklok/toolhive-catalog#1215, stacklok/toolhive-core#105, stacklok/toolhive#5273

Start by reviewing the listed legacy parsing paths, including pkg/registry/legacyhint/, convert.go, errors.go, upstream_parser.go, provider_remote.go, cmd/thv/app/registry_convert.go, and pkg/api/v1/registry.go with its tests. Resolve whether to retain the migration hint and conversion CLI, then verify no legacy imports or registry_legacy_format emissions remain. Run task test, task test-e2e, and task docs.