docs: trim prime to one screen

[DTTerastar]

Summary

The withings prime as merged was 143 lines (three terminal screens), in direct violation of CONTRACT §6:

Prime is short. It is not a man page. If it grows past one terminal screen, something belongs in this contract instead.

This trims it to 47 lines.

What came out

• Per-subcommand JSON key dumps (~35 lines). Replaced with a documented self-discovery line: ` --since 1d --format json`. An LLM agent can introspect the schema at runtime — it doesn't need a frozen copy in the primer.
• OAuth setup prose (~15 lines). The redirectmeto trick stays because it's the one non-obvious bit users need from the primer. The rest belongs in `auth login --help`.
• Two of the five examples. Kept the three with the highest signal-per-line.
• Gotchas tightened from multi-line paragraphs to single-line bullets.

Test plan

• `go build ./...` passes
• `withings-export prime` is 47 lines (was 143)
• All seven CONTRACT §6 sections still present (WHAT IT IS / I/O / AUTH / DATE FLAGS / SUBCOMMANDS / EXAMPLES / GOTCHAS)

Cross-repo

Same diet applied in parallel to crono and liftoff (their primes had the same problem at 123 and 131 lines respectively). Three matching PRs, separate per repo.

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