Add SSH Command Execution Tool for Coder Workspaces

This PR adds a new AI toolcoder_workspace_ssh_exec that allows executing commands in Coder workspaces via SSH. The tool provides functionality similar to thecoder ssh <workspace> <command> CLI command.

Key features:

• Executes commands in workspaces via SSH and returns the output and exit code
• Automatically starts workspaces if they're stopped
• Waits for the agent to be ready before executing commands
• Trims leading and trailing whitespace from command output
• Supports various workspace identifier formats:
  • workspace (uses current user)
  • owner/workspace
  • owner--workspace
  • workspace.agent (specific agent)
  • owner/workspace.agent

The implementation includes:

• A new tool definition with schema and handler
• Helper functions for workspace and agent discovery
• Workspace name normalization to handle different input formats
• Comprehensive test coverage including integration tests

This tool enables AI assistants to execute commands in user workspaces, making it possible to automate tasks and provide more interactive assistance.