v0.2.1: fixes from an outside review

Someone reviewed the code and found real gaps. These are the fixes.

• Secrets redacted. Arguments and results are redacted before they reach the decision log, the ledger file, the terminal prompt, the approval page or a scorer. Secret-looking names (password, token, api_key, card_number, ...) and known key formats (sk_live_, ghp_, AKIA, Bearer, JWTs) become [REDACTED]. Add your own with @gate.tool(..., sensitive={"field"}). The tool still gets the real values, and an edit that leaves [REDACTED] keeps the real value.
• Scope. Gate(scope="user-123") or gate.goal(text, scope=...) puts the user in the dedupe key, so two users sending the identical email through one shared ledger are two calls. No scope set means the same keys as v0.2.0.
• Expiry. Ledger rows and in-memory results older than the 24 hour window are now deleted (every 100 writes, or Ledger.prune()), not just ignored.
• Docs say what's measured. The README has a "What we know" table: each claim, its evidence, whether it's measured, true by construction or untested, and its limits. "Can't happen twice" wording is gone: it catches identical calls, and never retries an uncertain one automatically.
• 12 new tests (61 in total).

What's measured, what's true by construction and what's untested: see "What we know" in the README.