docs: clarify explicit schema validation direction

[FanouZeng-TT]

Description

FAQ.md says payloads without ucp.capabilities or meta.profile always hit
an error and must pass --request or --response:

| Neither | Error (must specify `--request` or `--response`) |

That is true for self-describing payloads, but explicit schema validation has a
different default. In the validate --schema ... path, the CLI falls back to
request direction when no direction flag is set:

determine_direction(request, response, None).unwrap_or(Direction::Request)

A minimal explicit schema validation with no payload metadata therefore succeeds
without either flag.

Fix: scope the table to self-describing payloads and document the explicit
--schema default, while keeping the existing note that resolving a plain schema
still requires an explicit direction.

Category (Required)

• Core Protocol: Protocol specification changes. (Requires Technical Council approval)
• Governance/Contributing: Governance or contribution process changes. (Requires Governance Council approval)
• Capability: Capability specification changes. (Requires Maintainer approval)
• Documentation: Documentation updates. (Requires Maintainer approval)
• Infrastructure: Build, CI, tooling, or release changes. (Requires DevOps Maintainer approval)
• Maintenance: Dependency, compatibility, or routine maintenance changes. (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. (Requires Maintainer approval)
• Community Health (.github): Repository-wide health files and configuration. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide.
• 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)

$ cargo run --quiet -- validate /tmp/payload.json --schema /tmp/schema.json --op create
Valid

$ cargo test validate_schema_local_base_accepted_with_explicit_schema --test cli_test
1 passed

$ uvx pre-commit run --all-files
trim trailing whitespace...Passed
fix end of files...Passed
check yaml...Passed
check json...Passed
check for added large files...Passed
detect private key...Passed
check that scripts with shebangs are executable...Passed
prettier...Passed
fmt...Passed
codespell...Passed

$ git diff --check
# no output