fix: never allow the projects directory to become a project directory

Loading a .gns3 placed directly in the projects root registered the
shared projects directory as the project path (load_project derives
the path from the file's parent directory). Deleting such an entry ran
rmtree on the projects directory itself, wiping every project until a
root-owned file stopped it, and left a zombie entry in the controller.

Three layers of protection:

- Controller.load_project() refuses a .gns3 whose parent directory is
  the projects directory; the normal subdirectory layout is unaffected
- the Project.path setter rejects the projects directory itself and
  its ancestors, closing the same hole for POST/PUT with an explicit
  path
- Project.delete() uses realpath + commonpath instead of commonprefix:
  entries whose path is the projects root are refused, and sibling
  directories sharing a string prefix (/srv/projects-evil vs
  /srv/projects) are no longer treated as inside the projects dir

Also removes the project_load MCP tool: loading by raw server
filesystem path is a footgun for automated clients; projects can still
be opened by project_id via the remaining tools.

tests/agent/mcp/test_tool_params.py

@@ -36,7 +36,6 @@ HANDLER_FILES = {
     "lock_project_handler": "projects.py",
     "unlock_project_handler": "projects.py",
     "get_locked_project_handler": "projects.py",
-    "load_project_handler": "projects.py",
     "get_nodes_handler": API_HANDLERS_FILE,
     "get_node_handler": API_HANDLERS_FILE,
     "start_node_handler": API_HANDLERS_FILE,