session: paginate message loading

[ry2009]

• Adds cursor pagination for session messages: GET /session/:sessionID/message?limit=…&before=… and sets Link:
  …; rel="next" when older pages exist.
  • TUI loads older messages when you scroll near the top (shows “Loading more…”), preserving scroll position.
  • Removes the hard 100 message trimming so loaded history doesn’t get dropped.
  • Regenerated SDK/OpenAPI (packages/sdk/*) and added a unit test packages/opencode/test/session/messages-
    pagination.test.ts.
  • Note: not true virtualization yet (we still render all loaded messages).

[ry2009]

maybe a separate pr ab implementing real windowing for true virtualization