feat: fetch --dry-run previews the price without paying

[reneaaron]

What

Adds fetch --dry-run: sends the request unpaid and reports the 402 challenge instead of settling it. Needs no wallet.

$ alby fetch --dry-run "https://www.l402apps.com/api/apis"
{
  "url": "https://www.l402apps.com/api/apis",
  "status": 402,
  "payment_required": true,
  "amount_in_sats": 10,
  "description": "L402 Apps — Get APIs directory"
}

• Price in sats is extracted from wherever the protocol puts the lightning invoice: WWW-Authenticate (L402 ... invoice= / MPP Payment ... invoice=) or a lightning-native x402's base64 Payment-Required header.
• When no lightning invoice is offered (e.g. a USDC-only x402 hit directly instead of through the l402.space bridge), the raw challenge is surfaced so the terms are still visible.
• A free endpoint reports payment_required: false.
• Payment flags (--max-amount/--currency/--unit/--network/--credentials) are rejected alongside --dry-run since a dry run never pays.

Why

From the review discussion on getAlby/payments-skill#28: prices listed on 402index.io can be missing, stale, or dynamic — the 402 challenge is the authoritative price at request time, but checking it previously meant hand-crafting a curl. Use case: comparing prices between two providers before paying.

Per that discussion this is deliberately not documented in the skill for now — the --max-amount default already guards automated flows.

Test

• yarn test green (111 pass, 2 skipped); 5 new unit tests cover L402, MPP, base64-x402 extraction, no-lightning-challenge fallback, and free endpoints (using a real BOLT-11 so decoding is exercised).
• Verified live against all three challenge shapes without paying: native L402 (10 sats), bridged x402 via l402.space (19 sats), lightning-native x402 (15 sats).

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