Add sample_random_scans script and first-page list-scans option

- scripts/sample_random_scans.sh: pick a random scan per machine (default: first list page) and download mosaic and/or tiles
- --list-scans-first-page-only: one HTTP request for scan list (up to 320 IDs)
- scripts/machines.example.txt; .gitignore local machines.txt (copy from example)
- README: document usage

scripts/machines.example.txt

@@ -0,0 +1,15 @@
+# All RootView minirhizotron machine labels (same set as `machine_metadata` in config.example.yaml).
+# Copy to the repo root as machines.txt, or: cp scripts/machines.example.txt machines.txt
+# sample_random_scans.sh: by default one random scan per line = mosaic + tiles; use MOSAIC_ONLY=1 for mosaics only
+BW1-4 [AMR-15]
+BW1-6 [AMR-19]
+BW1-7 [AMR-18]
+BW2-8 [AMR-25]
+BW2-10 [AMR-22]
+BW2-11 [AMR-23]
+BW2-13 [AMR-24]
+BW3-16 [AMR-16]
+BW3-17 [AMR-20]
+BW3-19 [AMR-21]
+BW3-20 [AMR-26]
+BW3-21 [AMR-17]