feat: add working_dir parameter to spawn tool for project-specific subagents

[GodGMN]

Summary

Adds a working_dir parameter to the spawn tool, allowing subagents to operate in specific project directories and automatically inherit project conventions from AGENTS.md as well as the SKILLS.

Problem

Previously, subagents always operated from the agent's workspace, making it difficult to:

• Run subagents in specific project directories
• Have subagents follow project-specific conventions (defined in AGENTS.md)
• Have subagents use SKILLS in the project directory
• Keep file operations isolated per project (sometimes they would put documentation and similar kindof files in the bot's base workdir, instead of the project itself)

Solution

• Added optional working_dir parameter to SpawnTool JSON schema
• Pass working_dir through to SubagentManager.spawn()
• Resolve and validate working directory paths
• Set file tool base directories and exec working directory to specified path
• Automatically load AGENTS.md from working_dir into subagent system prompt

The new parameter is completely optional. In case it isn't included, it just functions normally.

I have added to the AGENTS.md of my coding nanobot that every subagent spawned should include a workdir parameter. Seems like it works well for strictly structured projects that may have very specific workflow/architecture instructions in the AGENTS.md. Previously, subagents would missplace files somewhat often, and they never followed the more strict project conventions.