x402: optional `qscli topup --x402` (lazy deps, env-only key, hidden subcommand)

[raullenchai]

Goal

Optional, low-key qscli subcommand to fund a key over x402 (USDC on Base) —
so an autonomous/headless agent can self-provision inference credit without a
human opening a card. Mirrors the website's low-key treatment (see the x402 docs
page and backend #257): available to the agent/headless crowd, invisible to the
mainstream CLI user.

This is a fast-follow, not MVP. Anyone can already script the top-up today
with the standalone reference client (x402/deploy/pay.py pattern) against
POST https://pay.quicksilverpro.io/x402/topup/{1,5,10}. A CLI subcommand is a
convenience, not a gap.

Scope

• qscli topup --x402 [--amount 1|5|10] (or qscli x402 buy <amount>): runs
  the 402 → sign → retry handshake and prints/stores the minted key.
• Lazy dependency: put x402 + eth_account behind an extra —
  pip install quicksilverpro[x402] — and import them inside the
  subcommand only. The default install must not pull crypto deps or grow.
• Key handling (security-critical): read the payer wallet key from an env
  var only (e.g. QSP_X402_WALLET_KEY). Never persist it, never write it
  to config, never log it. This is the biggest risk surface — design first.
• Low-key surface: keep it out of the top-level --help for mainstream
  users (or gate visibility); no USDC/crypto wording in the default UX. Same
  anti-scam framing the website uses.
• On success, offer to write the returned sk-... into the usual CLI key
  config (the standard flow), so subsequent calls "just work".
• Docs: one section, link to https://quicksilverpro.io/docs/x402/.

Why gated / not now

• x402 top-up is niche today; most CLI users configure a key from the dashboard.
• Bundling a wallet/crypto surface into a mainstream LLM CLI risks the exact
  "looks scammy" perception we deliberately avoid on the site.
• Decide whether to build after the ecosystem listing lands (ecosystem: add QuickSilver Pro (Services/Endpoints) coinbase/x402#281)
  and we see real agent traffic on the x402 endpoint.

Related

• backend #257 (x402 Route A live + first payments), #258 (pipeline), #259
  (discoverability), ecosystem: add QuickSilver Pro (Services/Endpoints) coinbase/x402#281 (ecosystem listing).
• Live docs: https://quicksilverpro.io/docs/x402/