refactor: move API routers into domain subpackages
Moves 21 API router files into 6 domain subpackages (max 6 files each):
- api/tool/ — tool_instances, tool_types, tool_definitions, sessions
- api/config/ — config_profiles, user_config
- api/workspace/ — workspaces, workspace_files, workspace_git, workspace_instances
- api/user/ — users, auth, ssh_keys
- api/project/ — projects, git_repositories
- api/system/ — health, events, notifications, dashboard, terminal, instance_proxy
sessions_router extracted from tool_instances.py into tool/sessions.py.
main.py now imports from subpackage __init__.py re-exports.
Cross-router imports updated to use new paths.
Fixed pre-existing E712 in tool_definitions.py (is_base == False → is_(False)).
Quality gates: py_compile passed on all files, ruff passed.
alex
38c51ed95e