Execute dashboard issue query qualifiers

[espadonne]

Summary

• add sort: and review-requested: parsing/execution for issue and PR search
• route dashboard issue/PR lists through parsed query filters
• document dashboard query behavior

Tests

• GOCACHE=/private/tmp/shithub-qxx-dashboard-query-parity/.gocache go test ./internal/search ./internal/web/handlers
• git diff --check
• make lint-policy
• make lint-markdown
• GOCACHE=/private/tmp/shithub-qxx-dashboard-query-parity/.gocache make build

Notes

• GOCACHE=/private/tmp/shithub-qxx-dashboard-query-parity/.gocache make test is blocked in this sandbox by httptest local port binding, storage setgid repair, and DNS resolution for example.com.