[codex] Add scenario runner dbt step

[bill-ph]

Summary

• add a dbt_run scenario executor with dbt command sequencing, artifact capture, and classified failures
• add a frozen persons/events dbt project and posthog_frozen_dbt.yaml scenario
• document just scenario-frozen-dbt and DUCKGRES_SCENARIO_DBT_BIN

Testing

• GOCACHE=/tmp/duckgres-pr6-gocache go test ./tests/scenario/dbt ./tests/scenario -run "TestExecutor|TestFrozenDBT|TestProvisionSmoke"
• GOCACHE=/tmp/duckgres-pr6-gocache go test ./tests/scenario/... ./tests/perf/...
• just test-scenario
• just lint
• git diff --check

Live dev just scenario-frozen-dbt was not run from this checkout; it needs the real dev scenario env and local dbt installation.

[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	12	1	0
E2E/journey files	0	0	0
Workflow files	0	0	0

Signals

• Test cases: +3 / -0
• Assertions: +44 / -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.