mcpx: Phase 3 of #1675 — review-first mcpx elicit surface + nix#255 truth-up #1678

Phase 3 of design #1675 (mcpx elicitation broker hygiene). Design doc: files/designs/code-mcpx-elicitation-broker-hygiene.md in the keep-the-four-repos-healthy goal workspace. Gated on Phases 1–2 (the owner-only store and retention/events issues of #1675).

Scope

Make mcpx elicit a review-first answering surface, then tell the truth in the registers.

• URL questions (internal/cli/elicit.go renderQuestion ~L201-232): print the URL's registrable domain on its own emphasised line apart from the full URL (the full URL still prints — do not regress the covered show-full-url behaviour). If the host contains xn-- labels or non-ASCII that renders as punycode, print an explicit warning to check the domain is the one expected.
• Form questions: pre-fill the answer example (exampleFor ~L275-285, today "..." for every field) with each property's declared schema default where one exists; describeSchema (~L235-261) gains the default in the per-field detail. No default declared → "..." as today.
• Review step: mcpx elicit answer on an interactive terminal renders the parsed answer back against the question (field → value, with credential-shaped fields — the ones elicit.Route routes to humans — masked) and asks for confirmation before Broker.Respond relays it. An opt-out flag (name settled in triage, e.g. --yes) preserves one-shot behaviour for scripts; non-interactive use is byte-for-byte today's behaviour.
• Truth-up: retire the six elicit-ui conformance gaps (elicitation-app-review-modify-responses, elicitation-client-form-review-modify-responses, elicitation-client-url-show-domain-and-consent, elicitation-client-handle-urls-carefully, elicitation-client-highlight-domain, elicitation-client-warn-punycode) in internal/conformance/coverage_clientfeatures.go, and update the remaining #255-tagged gap texts that Phases 1–2 made stale (elicitation-server-follow-best-practices mentions world-readable storage).
• Post the completion evidence on mcpx: elicitation storage is readable by any local user and never pruned; elicitation UI and response validation rows dezren39/darwin-nix#255 — commenting, not closing: its answer-validation acceptance is only partly this design's (client slice is mcpx: Phase 3 of #1481 — validate elicitation form answers against requestedSchema in client.answer #1484; the server-relay slice elicitation-server-validate-received-data stays open and must be named as remaining in the evidence comment).

Evidence (main d39e611, 2026-10-28)

• Origin mcpx: elicitation storage is readable by any local user and never pruned; elicitation UI and response validation rows dezren39/darwin-nix#255 acceptance: "The answering surface shows which server is asking and allows review per the elicitation UI rows." (Server display already exists in renderQuestion — from <server>.<tool> — and is covered elsewhere; the open rows are review, defaults, and URL care.)
• Gap text for all six UI gaps: "mcpx elicit prints the raw request: no review step, no domain emphasis or punycode warning, no defaults pre-filled".
• Defaults precedent: the client-side path already applies schema defaults (mcpclient.TestElicitationDeclarations / TestModernElicitationAppliesSchemaDefaults, covered rows in the conformance register) — the CLI surface is the one place defaults are ignored.

Acceptance

• URL question render: domain on its own line; a punycode host produces the warning; a plain host does not.
• Form question render: fields with schema defaults show those defaults in the detail and in the answer example; fields without keep "...".
• Interactive answer: the parsed answer is rendered (credential-shaped values masked) and nothing is relayed until confirmed; declining the review relays nothing. With the opt-out flag, and non-interactively, behaviour matches today.
• Six elicit-ui gaps retired with tests/render fixtures as evidence; stale #255 gap texts updated.
• Evidence comment posted on mcpx: elicitation storage is readable by any local user and never pruned; elicitation UI and response validation rows dezren39/darwin-nix#255 naming what landed and what remains (server-relay validation slice).
• CI green for pkgs/mcpx (tests run in GitHub Actions).