Show draft failures with a consumer sentence and quotable refId

[TheGreatAxios]

Fixes CL-6749

A failed agent-create draft was answering { code, message } from a hand-wired hub route. The client only accepts { userMessage, refId }, so the panel showed a generic 422 with nothing to quote.

• Hub mounts createAgentDefinitionDraftRoutes so 422 uses makeErrorEnvelope
• Drafting failures go through reportError so the log and envelope share a refId
• ApiQueryError carries refId; postJSON uses parseErrorEnvelope
• The create-agent panel renders the consumer sentence plus Reference: <refId>

[TheGreatAxios]

Self-review

Verdict: the change is correct and well-tested — hub now serves the package's createAgentDefinitionDraftRoutes (canonical {code, userMessage, refId} envelope via reportError), and the client/panel surface the consumer sentence plus a quotable refId. No defects found; nothing needed fixing.

• Envelope plumbing (makeErrorEnvelope/reportError/parseErrorEnvelope, ApiQueryError.refId) — DISMISSED: checked signatures end-to-end, all call sites and the new optional constructor param are consistent; no exactOptionalPropertyTypes hazard.
• Hub route de-duplication (hand-wired 422 body removed, createAgentDefinitionDraftRoutes mounted instead) — DISMISSED: verified no dangling imports/usages of the removed error classes or the old agentDefinitionDraftingInFlightPrincipals set remain in apps/hub/src/index.ts.
• No bare catch {} / no as T on untrusted input / no fallback path left beside the new one — DISMISSED: checked, none present.

Targeted tests run (all pass):

• apps/web: bun test test/agents-api.test.ts test/create-agent-panel.test.tsx — 27 pass
• packages/agent-directory: bun test test/agent-definition-draft-routes.test.ts — 3 pass
• apps/hub: bun test src/draft-route-envelope.test.ts — 1 pass

[linear-code]

CL-6749