feat(agent): host collector for /proc loadavg, meminfo, uptime

agent/test/fixtures/proc/loadavg

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+0.42 0.55 0.31 3/512 12345

agent/test/fixtures/proc/meminfo

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+MemTotal:       16384000 kB
+MemFree:         2048000 kB
+MemAvailable:    8192000 kB
+Buffers:          256000 kB
+Cached:          4096000 kB
+SwapTotal:       4194304 kB
+SwapFree:        4194304 kB

agent/test/fixtures/proc/uptime

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+123456.78 987654.32

agent/test/proxmox_agent/collectors/host_test.exs

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+defmodule ProxmoxAgent.Collectors.HostTest do
+  use ExUnit.Case, async: true
+
+  alias ProxmoxAgent.Collectors.Host
+
+  @proc Path.expand("../../fixtures/proc", __DIR__)
+
+  test "collects load average" do
+    sample = Host.collect(proc_dir: @proc)
+    assert sample.load1 == 0.42
+    assert sample.load5 == 0.55
+    assert sample.load15 == 0.31
+  end
+
+  test "collects memory in bytes" do
+    sample = Host.collect(proc_dir: @proc)
+    assert sample.mem_total_bytes == 16_384_000 * 1024
+    assert sample.mem_available_bytes == 8_192_000 * 1024
+    assert sample.mem_used_bytes == sample.mem_total_bytes - sample.mem_available_bytes
+  end
+
+  test "collects uptime seconds" do
+    sample = Host.collect(proc_dir: @proc)
+    assert sample.uptime_seconds == 123_456
+  end
+
+  test "includes hostname string" do
+    sample = Host.collect(proc_dir: @proc)
+    assert is_binary(sample.hostname)
+    assert sample.hostname != ""
+  end
+
+  test "missing proc files yield :errors field, not a crash" do
+    sample = Host.collect(proc_dir: "/nonexistent/path/xyz")
+    assert sample.errors != []
+  end
+end