Add unified project agent workspace

[botlong]

Summary

• Add gateway protocol models/client for projects, agents, sessions, messages, and SSE events.
• Add Riverpod stores and agent chat adapters for OpenCode, Claude Code, and Codex through the gateway boundary.
• Replace the first tab with project-based navigation and add new conversation/session chat flows grouped by agent/model.

Verification

• docker run --rm -v \D:\Code\WorkSpace\remote-multi-agent:/workspace -v remote_multi_agent_pub_cache:/root/.pub-cache -w /workspace ghcr.io/cirruslabs/flutter:3.27.1 flutter analyze
• docker run --rm -v \D:\Code\WorkSpace\remote-multi-agent:/workspace -v remote_multi_agent_pub_cache:/root/.pub-cache -w /workspace ghcr.io/cirruslabs/flutter:3.27.1 flutter test --reporter expanded