Release v0.3.1: command kind and SDK cleanup - #13

Summary

Release v0.3.1 of the xfetch API/SDK.

• Adds the command world to wit/xfetch-runtime.wit for CLI command addons.
• Adds the xfetch-command-api crate with command request types and plain-text output helpers.
• Bumps all API crates to 0.3.1.
• Removes an unused direct serde dependency from xfetch-guest-api.
• Removes the unused parse_json helper from xfetch-command-api.
• Updates the changelog, protocol reference, WebAssembly guest guide, README, ROADMAP and the new docs/command-sdk.md.

No existing kind, world, protocol or public crate is removed. The JSON protocol remains version 1 and the WIT package remains xfetch:runtime@0.1.0.

Validation

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• cargo audit --deny warnings