[codex] Add review artifact metadata

[explicit09]

Summary

Implements issue #5 by adding a versioned ReviewArtifact metadata primitive to vedit-core for downstream review packages and signed reasoning trails.

Changes

• Add vedit_core::review_artifact::ReviewArtifact with schema vedit.review_artifact.1.
• Keep provenance fields optional for backward-compatible legacy or partial artifacts.
• Add builder-style helpers for render path, generated timestamp, source commit, timeline hash, tags, commit header, and reasoning body.
• Document the JSON serialization format in docs/ARCHITECTURE.md.
• Add serialization/deserialization tests for complete, legacy, and empty artifacts.
• Ignore project-local .worktrees/ directories.

Validation

• cargo fmt --all -- --check
• cargo test

Closes #5.