fix: move agent-os-kernel to optional dep, correct version constraint

[imran-siddique]

Root cause

\�gent-os-kernel>=4.0\ was declared as a hard dependency in \pyproject.toml, but:

1. Version 4.0 does not exist — latest is 3.7.0
2. \scanner.py\ imports it with \ ry/except ImportError\ and a full no-op fallback — it was never a hard requirement

This caused \pip install -e '.[dev]'\ to fail on every CI matrix combination, blocking all 6 jobs.

Fix

• Remove \�gent-os-kernel\ from required \dependencies\
• Add new [agt]\ optional extra for users who want AGT catalog scanning: \�gent-os-kernel>=3.0\
• Add \�gent-os-kernel>=3.0\ to [dev]\ so CI exercises the scanner code path

Test plan

• \pip install -e '.[dev]'\ succeeds on Python 3.11/3.12/3.13
• All 6 CI matrix jobs pass
• \pip install -e '.[agt]'\ works for users who want the AGT scanner

🤖 Generated with Claude Code

[imran-siddique]

Superseded by #121. Making agent-os-kernel optional is not viable — proxy.py has hard top-level imports from agent_os.* so the gateway fails to start without it. #121 uses agent-os-kernel>=3.7 as a hard dep (the correct version constraint) and also fixes the AGT 3.7 API incompatibilities in tests and schema.