docs: align bridge framing with revised single-dimension certification model

[bokelley]

Summary

Maintainer walked back the two-badge proxy-vs-state-local-seller split from #1782 and proposed a single-dimension framing: every seller faces the same verifiability gap; the bridge is one of two mechanisms for closing the seed→read loop, not a special path for one seller class. This PR folds the revised framing back into the JSDoc on AdcpServerConfig.testController (which currently names "upstream-proxy sellers" as the audience) and into skills/build-seller-agent/SKILL.md.

Also posted matching revision to the upstream taxonomy proposal at adcontextprotocol/adcp#4593.

Changes

JSDoc (src/lib/server/create-adcp-server.ts:1676-1707):

• Drops "only upstream-proxy sellers" as primary framing
• "Pick by where your read handlers fetch from, not by seller class"
• "Either path earns wire-conformance credit; it is not a separate certification category"

Seller-agent skill (skills/build-seller-agent/SKILL.md):

• New "Test surfaces — making your agent verifiable without live credentials" section between "Validate locally" and "Deployment"
• Frames the verifiability gap as universal, names both implementations (state-local store vs TestControllerBridge), explains the decision rule
• Hedges on certification names ("Wire Conformance" / "Live Integration Verified") while the badge model in Bridge leaderboard policy: how does a runner-visible bridge marker get consumed? #1782 settles

No SDK behavior change

The marker contract from #1786, the JSDoc trust-boundary section from #1787, and the construction-time dual-emit warn from #1788 all stay as-is — they describe mechanism without committing to a certification taxonomy. Only the framing prose around "what kind of seller this is for" shifts.

Test plan

• Doc-only change; tsc --noEmit clean
• prettier --check clean
• Patch-level changeset

🤖 Generated with Claude Code

[bokelley]

Docs review pass — folded in commit c687bc7

Docs-expert review caught one material edit and one minor:

Material: the new SKILL.md section had three paraphrases of the same decision rule (bullets, then symptom-form paragraph, then hedge blockquote) — an LLM-agent reader would spend tokens reconciling them and might tag the section "not actionable yet." Fixed:

• Collapsed the symptom-form paragraph into the bullets as inline diagnostics
• Shrank the hedge blockquote from three sentences to one ("Certification names in Bridge leaderboard policy: how does a runner-visible bridge marker get consumed? #1782 are under review; the mechanism here is stable.")
• Net: ~40% shorter, same actionable content.

Minor: JSDoc had no in-repo cross-link — IDE-hover readers had to jump to GitHub for the wiring mechanics. Added docs/guides/VALIDATE-YOUR-AGENT.md § "Platform-proxy sellers" as a primary anchor; the GitHub issue links demoted to "for the certification model under review."

Docs-expert verdict: lands-well; JSDoc rewrite was already materially better than what it replaced. SKILL.md hedge framing is now scoped to certification names, not mechanism.

CI re-running; admin merging once green.