feat(vibewatch-sentiment): --asset sBTC|STX picks the 402 option to pay

[sonic-mast]

Why

Four Tier 1 winners of the Stacks Vibe Index paid-query bounties (aibtc board mtt3jjrgcf0aa8fb225c, mttjxw940faa8df06de7, mtwd0kcs90cb2cfcf830, mtwd0cet93a16ab82498) and Vibewatch-io/vibewatch-mcp#16 asked for the same two things: a way to pay the index's advertised STX price, and docs saying that the 402 lists sBTC first. The engine's interceptor always paid the first stacks: option, so a wallet holding STX but no sBTC signed an sBTC transfer it could not fund, and the only way to pay in STX was to bypass the interceptor.

What

• src/lib/services/x402.service.ts — createApiClient(baseUrl, tool, { preferredAsset }). New exported selectStacksPaymentOption(accepts, preferredAsset?): no preference keeps the old first-Stacks-option rule; "STX" picks the first Stacks option whose asset is literally STX; "sBTC" picks the first one passing the builders' contract-id test. A preference the challenge does not offer rejects before anything is signed and names the offered assets; an invalid value rejects when the client is created (same place the payment mode is validated). Works in sponsored and direct modes; the selected option flows into the existing builders unchanged.
• vibewatch-sentiment — --asset sBTC|STX (default sBTC) on project, evidence, delta, validated before the free preflight and before any wallet access.
• SKILL.md / AGENT.md — a "Choosing the asset" / "Paying in STX instead of sBTC" section (402 lists sBTC first, engine default = first option, --asset STX to pay the STX option, select accepts[] by asset not by position), an error-table row for the unoffered-asset refusal, and one line on settlement latency (winners observed the paid payload under 60 s after broadcast).

Default behaviour is unchanged: without preferredAsset every existing caller pays exactly what it paid before.

Verification

• bun run typecheck, bun run validate (204/204) — pass.
• bun test src/lib — 160 pass, 0 fail, including 10 new tests: the pure selection rules, fake-server runs paying sBTC and STX in both orders and both modes (decoded tx is a ContractCall for sBTC and a TokenTransfer for STX, accepted.asset matches), the unoffered-asset refusal (no paid request, no Stacks API hit), and the invalid-value refusal at client creation.
• Local review gate (scripts/gemini-review.py --base origin/main): no findings.
• Not exercised: a live mainnet STX payment.

Related: a server-side change in vibewatch-app refuses an unscored panel slug with a free 422 before the 402, so the client-side --allow-unscored guard here stops being the only protection once that deploys.

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