Allow specifying working directory when spawning subagents via the Agent tool

Currently, subagents spawned via the Agent tool inherit the working directory of the parent session. This makes it impossible to orchestrate work across multiple unrelated projects (in different directories) from a single Claude Code session.

Requested: Add an optional cwd parameter to the Agent tool so the orchestrating session can specify which directory a subagent should start in.

Use case: Running a single orchestrator that delegates work to agents on different local projects simultaneously, without needing separate terminal sessions per project.