feat(x402): improve 402 html page and its prompts

[OisinKyne]

Problem to be solved

The prompts on the html payment required UI don't tell the user that an agent service probably needs a task to be parsed, and an inference service is designed to be given to obol buy inference for use as an agent's brain. This PR aims to increase the success of people copying and pasting the prompts into their agents.

Summary

Need to test a little more, but pushing to a pr to start, also opening another pr (#594) for swagger that's intended for 0.11

402 HTML page — type-aware copy
(internal/x402/paymentrequired.go

• template)

• New PaymentDisplay fields:
  OfferType, OfferName,
  OfferDescription, Model.
• buildTypeCopy produces a
  per-type lede + primary "Buy" card

• secondary prompt cards.
  buildMetaDescription now prefers
  the operator description for
  OG/Twitter previews.

• Inference offers get a primary
  obol buy inference --seller
  … --model … --budget 1
  --no-verify-identity shell snippet
  (rendered in < pre >), and lede
  framing makes it clear they're
  paying for remote model time, not
  an agent.
• Agent offers get a primary
  chat-completions request snippet
  with model/messages/X-PAYMENT
  filled in, and lede framing flags
  that agents have
  tools/skills/memory and need a
  real prompt.
• HTTP offers keep the existing
  single-prompt Obol-agent CTA.
• Description, when set, renders
  under the Service card and goes
  into the OG meta.

Route + offer plumbing

• RouteRule
  (internal/x402/config.go) gains
  OfferType and Model.
• serviceoffer_source.go now
  copies
  spec.registration.description into
  RouteRule.Description (was
  previously a "ServiceOffer " debug
  label) and sets OfferType +
  Model.

CLI

• --description is now the primary
  flag on sell
  inference|http|agent, with
  --register-description kept as an
  alias. Resume path
  (buildResumeGatewayArgs) emits
  --description. Tests updated.
• sell inference --model rejects /
  and suggests a -- replacement
• RouteRule (internal/x402/config.go) gains OfferType and Model.
• serviceoffer_source.go now copies spec.registration.description into
  RouteRule.Description (was previously a "ServiceOffer " debug label) and
  sets OfferType + Model.

CLI

• --description is now the primary flag on sell inference|http|agent, with
  --register-description kept as an alias. Resume path
  (buildResumeGatewayArgs) emits --description. Tests updated.
• sell inference --model rejects / and suggests a -- replacement (the
  LiteLLM wildcard bug the agent hit).

buy.py

• cmd_pay now accepts --timeout , defaults to 100s (Cloudflare
  free-tier tunnel cap), wired into urllib.request.urlopen.
• SKILL.md gets two new pitfall entries (timeout default, slash-in-model
  breakage).

Re: the openapi branch: I left it alone — the new fields are intentionally
small so they layer onto richer Swagger-derived copy later without conflict.
When oisin/openapi lands, the description field on the 402 page is the
natural seam to either deep-link to the spec or expand into a multi-card
breakdown.