Port `monitor` (Monitor V2) commands from the Go fork

[beengud]

Port monitor (Monitor V2) commands from the Go fork

Source: beengud/observe PR #6. Files: cmd_monitorv2.go, ot_monitorv2.go.
New CLI status: Partial. New CLI alert list/view can read alerts and shows associated monitor id/name/description, but there is no Monitor V2 management/preview.

Old command surface

observe list monitor [<name-substring>]            # searchMonitorV2 -> id, name, disabled, updatedDate
observe get monitor <id>                            # monitorV2 -> YAML incl. definition block
observe monitor preview-query <file.json>           # evaluateMonitorV2Source -> compiled OPAL pipeline + result schema
observe monitor preview [--workspace <id>] <file.json>   # previewMonitorV2 -> "Would fire: true/false" + sample alarm groupings
observe monitor alarms [--workspace <id>] [--monitor-id <id>] [--since <duration>] [--level <level>]  # searchMonitorV2Alarms -> id, monitorId, level, status, startTime, endTime

Flags: --workspace, --monitor-id, --since, --level (critical|error|warning|informational).

Behavior / API

• preview-query → evaluateMonitorV2Source; preview → previewMonitorV2; alarms → searchMonitorV2Alarms; list/get → searchMonitorV2/monitorV2.
• Input JSON is a MonitorV2Input (min: name, ruleKind, definition).

Acceptance

• list/get/preview-query/preview/alarms ported; --format support; tests ported. Consider whether to also add create/update (old fork relied on web for create).