session: v2 import rejects V1 export JSON and its own export file (SchemaError missing keys)

Summary

v2.0.4 session import fails on both V1's opencode export output and V2's own
session export output — the exporter omits keys the importer requires. V1
sessions created after installing V2 have no supported migration path, and
export → import round-trips fail.

Environment

• opencode 2.0.4 (opencode2, npm @opencode/cli), V1 1.18.31 (winget)
• OS: Windows 11 (build 10.0.26100.8655)
• Shell: pwsh 7.6.6
• Plugins: @slkiser/opencode-quota@latest

Repro

1. V1: opencode export <id> > s.json → V2: opencode2 session import s.json
   → SchemaError: Missing key ["info"]["location"] (V1 exports info.directory)
2. Fix info → Missing key [...]["state"]["error"] — importer requires
   state.error on every tool part
3. opencode2 session export <id> > v2.json, re-import → same state.error
   failure: exporter omits error on 58/59 tool parts, so V2's own export
   can't round-trip
4. Add error everywhere → Missing key [...]["state"]["content"][0] —
   exporter omits content on error-status tools, importer wants non-empty

Expected

Import accepts V1 export JSON, and V2's own export round-trips.

Notes

• Consistent, not intermittent. Same class as #21941 (fixed in 1.4.3, recurred here).
• Install-time V1 migration gap is #41217 — CLI import is the only path for
  sessions created after install.
• Import endpoint undocumented/experimental; migration guide has no session section.
• Worked around with a custom converter (V1 → v2 shape); 736 msgs / 570 tool
  parts imported fine after.