fix: modal support for local python modules

[ashenfad]

This PR adopts Modal's add_local_python_source for handling local Python module deps for the modal host.

[Copilot]

Pull request overview

This PR adds support for local Python modules in Modal deployments by utilizing Modal's add_local_python_source API. The implementation detects local packages (those not installed from PyPI), separates them from PyPI dependencies, and handles them appropriately during image building.

Key Changes:

• Adds detection logic to distinguish local modules from PyPI packages
• Implements optional dependency detection for packages with extras
• Integrates local package handling into Modal's image build process

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
agex/agent/registration.py	Adds _is_local_module() helper to detect local packages and _get_installed_optional_deps() to find installed optional dependencies; updates dependencies property to populate local_packages
agex/host/dependencies.py	Adds local_packages field to Dependencies dataclass and includes it in the ID hash for cache invalidation
agex/host/modal.py	Uses add_local_python_source() to mount local packages in Modal images after pip installs
tests/agex/test_dependencies.py	Adds test coverage for optional dependency detection functionality

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