Add rtk ctest filter (summarize on pass, keep failing-test output)

Summary

Add an rtk ctest filter. CTest output lists every test with pass/timing lines; on a green run the per-test detail is low-value, while on failure the failing test's captured output is what matters.

Why

rtk discover flagged ctest as an unhandled command: 41 invocations over 30 days (e.g. ctest -R "test_..." --output-on-failure), with no RTK equivalent.

Suggested behavior

• On all-pass: collapse to a summary (N/N passed, total time, slowest few), drop per-test Test #k: ... Passed lines.
• On failure: keep the failing tests' names and their captured stdout/stderr verbatim (respect --output-on-failure), drop passing-test noise.