refactor: organize API routers and services into subpackages

Service organization (19 files moved into 6 subpackages):
- services/instance/ — event_bus, health_monitor, lifecycle_hooks
- services/config/ — config_profile_resolver
- services/git/ — clone, git_operations, git_service
- services/build/ — docker_build, manifest_compiler
- services/terminal/ — terminal_manager, terminal_session
- services/shared/ — correlation, file_service, notification_service,
  permission_fixer, readiness_probe, ssh_keys, tunnel, workspace_manager

API router organization (16 files moved into 6 subpackages):
- api/tool/ — tool_instances, tool_types, tool_definitions,
  tool_types_validation, sessions (extracted from tool_instances)
- 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

Updated main.py imports and all __init__.py re-exports.
Sessions router extracted from tool_instances.py into api/tool/sessions.py.

Quality gates: py_compile passed, ruff passed.

apps/api/src/api/system/__init__.py

@@ -1 +1,17 @@
-"""System module."""
+"""System API routers module."""
+
+from src.api.system.dashboard import router as dashboard_router
+from src.api.system.events import router as events_router
+from src.api.system.health import router as health_router
+from src.api.system.instance_proxy import router as instance_proxy_router
+from src.api.system.notifications import router as notifications_router
+from src.api.system.terminal import router as terminal_router
+
+__all__ = [
+    "dashboard_router",
+    "events_router",
+    "health_router",
+    "instance_proxy_router",
+    "notifications_router",
+    "terminal_router",
+]