chore: design-proposal round marker (#19 SEP-2106, #39 client caching)

No SDK changes. Design proposals written to the project docs/ tree
(outside this worktree):
  - docs/2026-06-19-sep2106-design.md
  - docs/2026-06-19-client-caching-design.md
Report lines appended to logs/agent-reports/37-overnight.md.

Author: felixweinberger