Improve `--seed` flag help text + add explicit reproducibility example to README

[sqllocks]

Reproducibility is one of Spindle's most useful properties — same seed, same output, every time — but the --seed flag in the CLI only has a one-line help string, and the README doesn't show a "generate the same data twice" example.

What to do

1. Expand the --seed help text in cli/main.py (or wherever it lives) to mention:
   • Default behavior when not provided
   • That the seed propagates through all generators in the run
   • That the same seed produces identical output (assuming same version)
2. Add a 6–8 line "Reproducibility" section to README.md under Quick Start with a worked example showing two runs of the same --seed producing identical row counts and the same hash of the first 100 rows
3. Optional: link to a tutorial page if you want to expand further

Why this is a good first issue

• Touches only docs + a help string — no engine changes
• Easy to verify (run the example, check it works)
• High user-visibility — first thing newcomers test

Feel free to suggest wording in a draft PR.