feat(terminal): add CLASS column to findings table + alert-fallback in METRIC

[cicdteam]

Summary

Two coupled UX improvements to the terminal scan table.

1. New CLASS column

Between SEVERITY and METRIC, showing each finding's stable class slug
(hot-label, label-pattern-overly-granular, never-firing-alert,
always-firing-alert, unused-metric, dashboard-sprawl).

The row is now self-identifying: severity tells importance, class tells type,
and the Reference: link in the detail block resolves directly to the
matching page at remetric.dev/findings/<class>. Width grows ~20 chars but
still fits a 132-col terminal.

2. Alert-finding fallback in METRIC column

Previously the alerthygiene analyzer's findings rendered with METRIC empty
(no metric exists for an alert rule). The CLASS column would say
never-firing-alert and the row showed empty METRIC, empty LABEL, 0 SERIES,
0 UNIQUE - looking like missing data.

Now when Metric is empty and Alert is set, the alert rule name (e.g.
RemetricNeverFired) renders in the METRIC column. The CLASS column tells
the reader the value is a rule name, not a metric name. No new ambiguity:
alert rule names are PascalCase, metric names are snake_case.

Empty METRIC for label-pattern-overly-granular findings stays empty -
those describe a global label, not per-metric, so empty is correct.

Sample (before / after)

Before:
│ MEDIUM   │                              │ user_id  │ 500    │ 500    │ ~499  │
│ MEDIUM   │                              │ trace_id │ 500    │ 500    │ ~499  │
│ MEDIUM   │                              │          │ 0      │ 0      │ ~0    │

After:
│ MEDIUM   │ label-pattern-overly-granular │                    │ user_id  │ 500 │ 500 │ ~499 │
│ MEDIUM   │ label-pattern-overly-granular │                    │ trace_id │ 500 │ 500 │ ~499 │
│ MEDIUM   │ never-firing-alert            │ RemetricNeverFired │          │ 0   │ 0   │ ~0   │

What changed

• internal/output/terminal/findings.go: add CLASS column header + cell; Metric fallback to Alert.
• internal/output/terminal/findings_test.go: Class: on each sampleFindings entry + new TestRenderFindings_ClassColumnAndAlertFallback regression test.
• internal/output/terminal/testdata/findings_all_severities.golden: regenerated with the new column.
• docs/getting-started.md: example output updated; "Reading the report" describes the CLASS column and the per-finding METRIC semantics.
• demo/remetric.gif: regenerated against demo stack with the new binary.

Test plan

• make fmt vet lint vuln clean.
• go test -race ./... clean.
• Real e2e scan output spot-checked for both happy-path and alert findings.
• mkdocs build --strict clean.
• CI green.
• Post-merge: docs.yml redeploys; gif refreshed on README.