CLI --load: opt-in error-rate / latency thresholds that set the exit code (for CI) #2745

Follow-up to #2439 / #2743.

Problem

A --load run that completes exits 0 whatever its error rate (documented in the CLI README). To use it as a CI gate today you have to parse the summary with --format json and jq and do the comparison yourself.

Proposal

Add an opt-in threshold so the exit code carries the verdict, for example:

• --load-max-error-rate <0..1>: fail when errorRate is above the value (it is already the share of calls that were tool errors or thrown errors).
• Possibly --load-max-p99 <ms> for a latency SLO. Decide whether that belongs in the same change.

When a threshold is breached, still print the full summary, then exit with a new dedicated exit code (or an existing one if one fits; check core/cli exit codes) and a structured error code naming which threshold failed. With no threshold set, behavior is unchanged.

Validation: the threshold flags require --load, and the value must be in range.

If #2744 lands first, add the flags there too, since both clients share core/cli/handlers/load-test.ts.

Priority score

Severity 2 (easy workaround with jq) + urgency 3 (wanted this milestone) + 1 (milestoned) = 6 → Medium.