feat(cli): add --profile flag to report slowest tests

[Chemaclass]

Goal

Add a --profile flag that reports the slowest N tests after a run.

Why

No per-test timing visibility today. Unlocks DX (find slow tests) and future perf work (measure before optimize).

Notes

• Runner already times tests; src/clock.sh exists.
• Print slowest N (default e.g. 10) sorted descending.
• TDD: RED → GREEN → REFACTOR. Bash 3.0+ compatible. Update help text + CHANGELOG.

Part of #677.