fix(pricing): an unedited copy of the template can no longer price a run at $0

[TMHSDigital]

Fixes #28.

docs/pricing.example.json shipped 0.0 for every price. The file's own note says 0 is the claim that tokens are free, so a copy used unedited priced every call at nothing, and --max-cost-usd let any run through.

What changed

• The template: prices are now null and as_of is the placeholder YYYY-MM-DD. The loader refuses that placeholder with a message saying what to fill in, so an unedited copy cannot price a run at all.
• The loader (config.py):
  • An entry with both prices at exactly 0 is refused unless it carries "free": true, because that is also what a half-filled template looks like.
  • "free": true alongside a nonzero price is refused as contradictory.
  • One price at 0 is still an ordinary price.
• README and PLAN say the copy is refused until it is filled in.

Checked

• The new tests failed on main and pass now:
  • the template is refused, and the message names the placeholder;
  • a filled-in copy loads;
  • 0/0 is refused;
  • 0/0 with free: true loads and costs 0;
  • one side at 0 prices normally.
• plumbline run tiny.jsonl --adapter mock --pricing docs/pricing.example.json exits 1 with the placeholder message, before any call.
• ruff, mypy --strict, and the full pytest suite pass, and the prose checks are clean.

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