Consume resolved sandbox tool policy

[chubes4]

Summary

• Replace WP Codebox's generated Data Machine sandbox ability list with a generic wp-codebox/sandbox-tool-policy/v1 snapshot contract.
• Require requested allowed_tools to validate against the caller-provided resolved snapshot before host or browser sandbox launch, failing closed with structured diagnostics when missing or denied.
• Pass sandbox-visible runtime tool IDs into the sandbox agent contract without interpreting Data Machine-specific taxonomy.

Tests

• npm run build
• npm run sandbox-tool-policy-smoke
• npm run agent-sandbox-code-smoke
• npm run task-input-contract-smoke
• npm run wordpress-plugin-smoke

Verification limitation

• npm run artifact-contract-smoke timed out after 5 minutes twice in the local worktree without producing a failure message.

Closes #523

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implemented the generic sandbox policy snapshot plumbing/enforcement, updated smoke coverage/docs, and ran local verification for Chris to review.