feat(trail): add --debug-dir to tune for inspecting prompt and response

entire trail tune --debug-dir <dir> writes the assembled prompt (prompt.txt) and, in --run mode, the raw model response (response.txt) to <dir>. This makes it possible to debug why a runner was skipped or tailored oddly: prompt.txt shows exactly what signal the model received (the gathered-context block), response.txt shows exactly what it returned before validation/parsing. Writes are best-effort — failures warn rather than abort the run.