fix: switch ramdisk from /dev/ram0 to tmpfs, refactor snapshot-download.py

The /dev/ram0 + XFS + format-ramdisk.service approach was unnecessary
complexity from a migration confusion — there was no actual tmpfs bug
with io_uring. tmpfs is simpler (no format-on-boot), resizable on the
fly, and what every other Solana operator uses.

Changes:
- prepare-agave: remove format-ramdisk.service and ramdisk-accounts.service,
  use tmpfs fstab entry with size=1024G (was 600G /dev/ram0, too small)
- recover: remove ramdisk_device var (no longer needed)
- redeploy: wipe accounts by rm -rf instead of umount+mkfs
- snapshot-download.py: extract download_best_snapshot() public API for
  use by the new container entrypoint.py (in agave-stack)
- CLAUDE.md: update ramdisk docs, fix /srv/solana → /srv/kind/solana paths
- health-check: fix ramdisk path references

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -1,3 +1,4 @@
 .venv/
 sessions.duckdb
 sessions.duckdb.wal
+.worktrees