[feature] sp export/import — move the store between machines

[rwrife]

Pitch

Backlog item #12. The store is local files by design, but people have more than one machine. Add a dead-simple, dependency-free way to snapshot and restore the whole store as a single tarball.

Behavior

• sp export [--out FILE] [--include-morgue] — write a .tar.gz containing the index + scratch files (and optionally the morgue). Defaults to scratchpatch-export-<timestamp>.tar.gz in cwd, or stdout with --out -.
• sp import <FILE> [--merge|--replace] — restore from a tarball.
  • --merge (default): add incoming scratches; on id collision, keep existing and report skips (never clobber silently).
  • --replace: back up current store to the morgue-adjacent backup, then replace. Must be explicit; two-step/destructive-safe per the project's design rule.
• Use stdlib archive/tar + compress/gzip only — no new deps.

Acceptance criteria

• sp export and sp import commands registered on root.
• Export round-trips: export → fresh store → import reproduces scratches + metadata identically.
• Import --merge never overwrites existing ids; reports collisions/skips.
• Import --replace is explicit and non-destructive by default (backs up before replacing).
• --out - streams the tarball to stdout; import reads from a path (and - for stdin if easy).
• Unit tests: export/import round-trip, merge collision handling, empty-store export.
• README updated with export/import usage and the merge-vs-replace note.

[rwrife]

Opened #36 implementing export/import. Phase 1 next cycle will merge once CI is green.