[codex] Add frozen metadata scenario

[bill-ph]

Summary

Adds the dev scenario runner coverage for the frozen metadata exploration path.

• add a posthog_frozen_metadata scenario that provisions a temporary warehouse, creates DuckDB views over a frozen dataset URI supplied by environment, runs metadata/catalog queries, and deprovisions the warehouse
• add SQL file and catalog-file support needed by the scenario, including ${env:NAME} template resolution and min_rows assertions
• make scenario execution path-stable when launched through scripts/scenario_run.sh / just scenario-frozen-metadata
• update the scenario runner runbook and mark the old prod perf harness path as legacy while this dev runner replaces it

Validation

• go test ./tests/scenario/...
• go test ./tests/scenario/... ./tests/perf/...
• just test-scenario
• just lint
• git diff --check

I did not run the live dev frozen scenario in this checkout because the live scenario API/PG environment is not configured here. The scenario also requires the provisioned Duckgres worker role to have read/list access to the configured frozen dataset prefix.

[github-actions]

Test Impact Plan

Deterministic summary of how this PR changes tests, CI runners, and coverage-risk signals.

Summary

Area	Added	Changed	Deleted
Test files	4	5	0
E2E/journey files	0	0	0
Workflow files	0	0	0

Signals

• Test cases: +9 / -0
• Assertions: +39 / -0
• Skips or known failures added: 0
• Workflow continue-on-error added: 0
• Workflow path filters added: 0
• Test commands removed from justfile: 0
• E2E/journey retry lines added: 0

Coverage risk: neutral or increased

No coverage-reduction warnings detected.