[product-manager-impersonator] Preview matching workflow runs before log download

## Feature Idea

**Summary:** Add a `--list-only` mode to `scripts/fetch-workflow-logs.py` that previews matching workflow runs (and writes metadata-only manifest output) without downloading log archives.

## Why a Customer Would Want This
When maintainers are debugging noisy CI periods, they often need to confirm **which runs** match `--since/--until/--conclusion` before pulling ZIP logs. Today the script always downloads logs once matches are found, which makes quick investigation loops slower and more expensive.

## Rough Implementation Sketch
- Add `--list-only` (and optional `--format table|json`) CLI flags in `scripts/fetch-workflow-logs.py`.
- Reuse existing `_fetch_runs(...)` output to print a compact run summary (`run_id`, `created_at`, `conclusion`, `html_url`) and skip `_download_runs(...)` when list-only is enabled.
- Keep manifest output behavior, but in list-only mode write run metadata with empty `log_files` arrays.
- Extend `tests/test_fetch_workflow_logs.py` with coverage for list-only path (no download call, correct manifest/screen output).

## Why It Won't Be That Hard
This is a focused change in one script plus one test module. The script already has clear phase boundaries (`_fetch_runs`, `_download_runs`, `_write_manifest`) and existing tests around filtering behavior, so list-only is mostly a control-flow addition rather than new infrastructure.

## Evidence
- `scripts/fetch-workflow-logs.py` currently always downloads when runs exist (`main()` calls `_download_runs` unconditionally after `_fetch_runs`, lines 200-213).
- The download phase is already isolated in `_download_runs(...)` (`scripts/fetch-workflow-logs.py`, lines 171-190), making it straightforward to bypass for preview mode.
- Existing test coverage for run filtering and fetch behavior is in `tests/test_fetch_workflow_logs.py` (lines 17-97), providing a natural place to add list-only assertions.
- Recent related activity shows active operator pain in this script’s filtering path: issue #1236 and PR #1237.

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