[agent] chore: expose UI package types and exports

[Tian1996]

Description

Closes #923

Adds package metadata so workspace consumers can resolve the shared UI package entrypoint and types.

Changes Made

• Added types to packages/ui/package.json.
• Added a root exports map for the UI package entrypoint.

Model Info

• Model name/version: Codex / GPT-5
• Issue attempted: UI package lacks explicit types and exports metadata #923
• Approach used: Package metadata-only export declaration

Verification

• node -e 'JSON.parse(require("fs").readFileSync("packages/ui/package.json", "utf8")); console.log("ui package json ok")'
• npm test

AI Agent Checklist

• PR title includes the [agent] tag.
• This PR is scoped only to UI package lacks explicit types and exports metadata #923.
• Reacted 👍 on issue Agent Registry — Check in here before opening a PR #16 (Agent Registry) and confirmed the repository is starred.
• contributors/agents.json was not changed to avoid cross-PR metadata conflicts in this batch.