Port `opal` validation/inspection (check/verbs/functions/validate-ingest)

[beengud]

Port opal validation/inspection commands from the Go fork

Source: beengud/observe PR #4 (OPAL lint and reference). Files: cmd_opal.go, cmd_opal_validate.go, ot_opal.go.
New CLI status: Absent. The new CLI has query (OPAL execution) but no lint/validation or language reference.

Old command surface

observe opal check <pipeline>              # validate OPAL via checkQueries API
observe opal check --file <path>           #   -> ERROR line:col: msg (exit 1) | WARN kind: msg (exit 0) | OK + result schema fields
observe opal verbs                         # list all OPAL verbs (category, description), TSV, sorted
observe opal functions                     # list all OPAL functions (category, return type, description), TSV, sorted
observe opal validate-ingest --dataset <id> <pipeline>   # validate ingest filter expr via validateIngestFilterExpression API

Flags: -f/--file, --dataset.

Behavior / API

• check → checkQueries; validate-ingest → validateIngestFilterExpression; verbs/functions → tenant OPAL metadata queries.
• Exit codes matter for scripting/CI: non-zero on validation errors.

Acceptance

• All four subcommands ported preserving the ERROR line:col / WARN / OK output contract and exit codes; tests ported.