Add agent_version label to node_info metric

[gecube]

Problem

The node_info metric documentation lists three labels — hostname, kernel_version and agent_version — but the agent only ever emitted the first two. The agent_version label was never present, as noted in the comment on #215.

This closes the documentation/implementation gap and gives the backend the data it needs to surface the agent version in the UI and to warn when an agent falls behind (the feature requested in #215).

Change

• Add the agent_version label to the node_info metric definition (appended last, so existing label positions are unchanged).
• Populate it with the build-time version (flags.Version, injected via -ldflags -X), passed through node.NewCollector.
• Update both the Linux and Windows collectors, which each emit node_info, so the positional label count stays consistent.

No new metric is introduced: the node_info info-metric is the contract the docs already promise, and its cardinality only changes across upgrades — the standard info-metric pattern.

Verification

Unit test (node/collector_agent_version_test.go) asserting the label is present with the expected value:

=== RUN   TestNodeInfoIncludesAgentVersion
--- PASS: TestNodeInfoIncludesAgentVersion (0.00s)
PASS
ok  	github.com/coroot/coroot-node-agent/node	0.002s

End-to-end: image built with --build-arg VERSION=v1.99.0-agentver-test, run on a live host, /metrics scrape:

node_info{agent_version="v1.99.0-agentver-test",hostname="orbstack",kernel_version="7.0.11-orbstack-00360-gc9bc4d96ac70",machine_id="82ddc538b9e95ae7c1d309201ccb5f9d",system_uuid=""} 1

The agent_version label matches the build-injected version, consistent with the existing node_agent_info{version=...} series.

Fixes #215

[def]

@gecube could you please stop bombing the Coroot repos with AI-generated PRs like this one? It doesn't solve any real problem and doesn't help the project.