feat(host_agent): collect network, disk I/O, per-core CPU, temps, memory PSI

Extends the push agent (v1.2.0) with the Netdata-style signals the operator
wants: per-core CPU, per-interface network throughput and per-disk I/O
(rates derived in-agent from monotonic /proc counter deltas, with
counter-reset clamping), hardware temperatures (/sys/class/hwmon),
memory-pressure PSI (/proc/pressure), and cached/buffers memory breakdown.
All stdlib-only.

Server side needs no migration — these land as additional rows in the shared
PluginMetric table via _expand_sample_to_metrics. Per-resource series use a
':' in resource_name (host:net:eth0, host:core0, host:temp:Package) so the
existing fleet widget's ':' filter ignores them; host-level totals/max are
emitted at the bare host resource. New sample keys are optional, so older
agents keep ingesting unchanged.

Unit tests for the new parsers, rate/reset logic, and the expander contract.
Data foundation for the Netdata-style host view (task #867).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugins/host_agent/plugin.yaml

@@ -1,7 +1,7 @@
 # plugins/host_agent/plugin.yaml
 name: host_agent
-version: "1.1.0"
-description: "Remote Linux host resource monitoring via a lightweight Python push agent (CPU, memory, storage, load, uptime)"
+version: "1.2.0"
+description: "Remote Linux host resource monitoring via a lightweight Python push agent (CPU incl. per-core, memory + PSI, storage, disk I/O, network throughput, load, temperatures, uptime)"
 author: "Steward"
 license: "MIT"
 min_app_version: "0.1.0"