Support hooks, skills, agents, instructions/rules from additional directories

https://github.com/github/copilot-sdk/pull/2180

Currently the adding folders via SDK or `--add-dir` only tells the model of the directories
Skills, Custom Agents, MCP servers, plugins, and the like aren't loaded from the additional directories
Claude supports loading of Skills and Agents from the additional directories
& also supports loading rules/instructions based on an Env variable

Start with the SDK and `--add-dir` entry points, then read the behavior described in pull request #2180. Identify how additional directories are currently exposed to the model and determine the expected handling for skills, custom agents, MCP servers, plugins, and rules or instructions from an environment variable. Done means these resources are loaded from additional directories as described.

Written by the indexing model from the issue text.