Rename to AgentRelay and add multi-project workspace mode

[arumwu]

What changed

• rename the product, npm package, CLI, MCP server, storage paths, and documentation to AgentRelay
• migrate legacy .devrelay/devrelay.db storage automatically without changing project identity
• preserve SQLite WAL and shared-memory sidecars during migration
• allow AgentRelay to use a fixed non-Git workspace boundary
• associate each agent session with a working directory inside that workspace
• resolve Git state from the child repository containing that directory
• prefix handoff file paths relative to the workspace
• migrate existing SQLite databases with a working_path session field
• add CLI and MCP inputs for working-directory context
• add the new product slogan to the README and package metadata

Why

The primary local directories are multi-project workspaces rather than Git repositories. AgentRelay coordinates Codex, Claude Code, and other agents across those child projects without making them repeat prior work.

Impact

Codex and Claude Code can share one AgentRelay instance per top-level workspace while retaining child-repository Git context. Existing DevRelay data is migrated on first startup, and working directories outside the configured workspace are rejected.

Validation

• npm run check
• 4 test files, 9 tests passed
• npm pack --dry-run
• initialized and migrated both real non-Git workspace roots successfully