feat(agent): toml config loader with defaults and validation

agent/test/fixtures/agent.toml

@@ -0,0 +1,8 @@
+server_url = "wss://monitor.example.com/socket/websocket"
+token = "test_token_123"
+host_id = "pve-test-01"
+
+[intervals]
+fast_seconds = 15
+medium_seconds = 120
+slow_seconds = 600

agent/test/proxmox_agent/config_test.exs

@@ -0,0 +1,62 @@
+defmodule ProxmoxAgent.ConfigTest do
+  use ExUnit.Case, async: true
+
+  alias ProxmoxAgent.Config
+
+  @fixture Path.expand("../fixtures/agent.toml", __DIR__)
+
+  describe "load/1" do
+    test "parses required fields" do
+      assert {:ok, cfg} = Config.load(@fixture)
+      assert cfg.server_url == "wss://monitor.example.com/socket/websocket"
+      assert cfg.token == "test_token_123"
+      assert cfg.host_id == "pve-test-01"
+      assert cfg.fast_seconds == 15
+      assert cfg.medium_seconds == 120
+      assert cfg.slow_seconds == 600
+    end
+
+    test "returns error for missing file" do
+      assert {:error, {:file_read, _}} = Config.load("/does/not/exist.toml")
+    end
+
+    test "defaults host_id to system hostname when absent" do
+      tmp = Path.join(System.tmp_dir!(), "agent_nohost.toml")
+
+      File.write!(tmp, """
+      server_url = "wss://x/socket/websocket"
+      token = "t"
+      """)
+
+      on_exit(fn -> File.rm(tmp) end)
+
+      assert {:ok, cfg} = Config.load(tmp)
+      assert is_binary(cfg.host_id)
+      assert cfg.host_id != ""
+    end
+
+    test "applies default intervals when [intervals] is absent" do
+      tmp = Path.join(System.tmp_dir!(), "agent_nointervals.toml")
+
+      File.write!(tmp, """
+      server_url = "wss://x/socket/websocket"
+      token = "t"
+      host_id = "h"
+      """)
+
+      on_exit(fn -> File.rm(tmp) end)
+
+      assert {:ok, cfg} = Config.load(tmp)
+      assert cfg.fast_seconds == 30
+      assert cfg.medium_seconds == 300
+      assert cfg.slow_seconds == 1800
+    end
+
+    test "returns error when required keys missing" do
+      tmp = Path.join(System.tmp_dir!(), "agent_bad.toml")
+      File.write!(tmp, "token = \"t\"\n")
+      on_exit(fn -> File.rm(tmp) end)
+      assert {:error, {:missing_key, :server_url}} = Config.load(tmp)
+    end
+  end
+end