docs: separate VerifyAgent product/demo from SDK and clarify SDK scope

[GsCommand]

Motivation

• VerifyAgent is now a separate public MIT repo and should not be presented as an in-repo product/UI demo in the SDK documentation.
• The SDK must be clarified as a reusable toolkit for receipt creation, canonicalization, hashing, signing, verification, ENS helpers, and agent-wrapping utilities.

Description

• Rewrote and cleaned docs to remove product/demo framing and to position VerifyAgent as an external public verifier, adding the line For public paste-and-verify receipt verification, use VerifyAgent: https://github.com/commandlayer/verifyagent.
• Updated root and package docs, including README.md, CHANGELOG.md (added Unreleased entry), QUICKSTART.md, EXAMPLES.md, typescript-sdk/README.md, python-sdk/README.md, and python-sdk/docs/{getting-started.md,verification.md,client.md} to reflect SDK-only scope.
• Did not remove any core SDK verification, canonicalization, hashing, signing, or ENS helper code; no product/demo application files were deleted.

Testing

• Verified remaining mentions via repository search with rg to ensure all VerifyAgent occurrences are external references only, which passed.
• Ran TypeScript package typecheck with cd typescript-sdk && npm ci && npm run typecheck, which completed successfully.
• Ran Python verification tests with python -m pytest python-sdk/tests/test_verify.py, all tests passed.

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Codex Task