docs: fix README schema URLs to versioned 2026-04-08 path

[FanouZeng-TT]

Description

Five https://ucp.dev/schemas/... URLs in the README return 404 — the published site
serves schemas under a versioned prefix (/2026-04-08/schemas/...). The already-merged
PR #57 fixed the resolve command example's URL; this PR covers the remaining unpatched
locations:

Location	Before	After
--schema-remote-base example (L123)	https://ucp.dev/schemas	https://ucp.dev/2026-04-08/schemas
Example payload schema field (L380)	https://ucp.dev/schemas/shopping/checkout.json	/2026-04-08/schemas/shopping/checkout.json
Example payload schema field (L384)	https://ucp.dev/schemas/shopping/discount.json	/2026-04-08/schemas/shopping/discount.json
$id demonstration (L410)	https://ucp.dev/schemas/shopping/discount.json	/2026-04-08/schemas/shopping/discount.json
$id demonstration (L488)	https://ucp.dev/schemas/shopping/fulfillment.json	/2026-04-08/schemas/shopping/fulfillment.json

All five fixed URLs return HTTP 200; the unresolved URL table in the tool output
section (L552–618) is left unchanged — it is actual CLI output, not authorial prose.

Category (Required)

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool (resolver, linter, validator). (Requires Maintainer approval)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

Screenshots / Logs (if applicable)

N/A — the before/after URL table and curl exit codes are the repro.