Run dev scenarios against isolated stacks

[bill-ph]

Summary

• make scenario-dev build and test an isolated Duckgres stack by default
• add a scenario-runner image and Kubernetes Job launcher for in-cluster scenario runs
• add an explicit use_shared_dev workflow input for deployed-dev checks
• update the runbook and workflow/script tests for the new execution model

Testing

• go test -count=1 ./tests/scenario ./tests/scenario-dev ./tests/e2e-mw-dev
• bash -n tests/scenario-dev/run.sh
• just lint

Note: actionlint is not installed locally, so it was not run.

[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	3	3	0
E2E/journey files	0	0	0
Workflow files	0	1	0

Signals

• Test cases: +8 / -1
• Assertions: +20 / -2
• Skips or known failures added: 1
• 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: likely reduced

Warnings

• New skips or known failures (likely reduced)
  • 1 skip/allowlist line(s) added