feat(alertd): report cpu usage on windows in the load check

[dannash100]

The `load` check skips on Windows, so Windows hosts send Canopy no CPU signal at all.

• On Windows it samples global CPU usage over 5s and reports `cpu_pct` in the check details
• Warns at 90% or above, never fails: one 5s sample is too noisy to fail on
• Linux keeps the load-average path unchanged

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🦸 Review Hero Summary (round 1) · reviewed 53554ae
3 agents reviewed this PR | 3 failed | 0 critical | 0 suggestions | 0 nitpicks | Filtering: consensus 3 voters, 2 below threshold

No issues found. Looks good!

Below consensus threshold (2 unique issues not confirmed by majority)

Location	Agent	Severity	Comment
crates/alertd/src/checks/load.rs:83	Performance	suggestion	cpu_usage() blocks the check for a full 5s (CPU_SAMPLE) via tokio::time::sleep. Since sweep.rs awaits all checks before returning (run_checks_concurrently), this adds a fixed 5-second floor to ever...
crates/alertd/src/checks/load.rs:94	Bugs & Correctness	suggestion	cpu_usage() only ever returns Check::warning or Check::pass — there's no Check::fail path, unlike the Unix load-average branch which escalates to Fail (is_fatal()==true, drives overall failing/pagi...