feat: add agent result recall

[roeyazroel]

Summary

• add opt-in persistence for completed agent results
• add safe legacy history migration and searchable agent-result records
• add Ctrl-R recall tabs for Commands and Agent results
• keep explanations copy-only and command insertion non-executing

User impact

With [history] agent_results_enabled = true, completed full, pipe, and conversation turns can be found in Ctrl-R recall. Tab and Shift-Tab switch visible Commands and Agent results tabs.

Validation

• rtk go test ./internal/config ./internal/history ./internal/shell
• rtk go test ./...
• rtk go vet ./...
• rtk go test -race ./internal/history

The combined history/shell race command still reports existing chzyer/readline close/startup failures outside this change.