Allow private agents to contact company agents

[yaojin3616]

Summary

• Allow private agents to discover and contact company-visible agents.
• Prevent private-to-private and company-to-private contact in the directory and message authorization paths.
• Add coverage for the communication matrix and private-agent directory query.

Root cause

The existing visibility rule exposed same-owner private agents and excluded company-visible agents for private sources, which contradicted the requested one-way communication policy.

Validation

PYTHONPATH="$PWD" uv run pytest tests/test_agent_visibility.py tests/test_query_directory_tool.py tests/test_agent_directory_api.py

PYTHONPATH="$PWD" uv run ruff check app/core/permissions.py app/services/agent_directory.py tests/test_agent_visibility.py tests/test_query_directory_tool.py

Closes #838