test: promote force and rollback runtime e2e scenarios (U6)

[joshua-temple]

Problem

Two promote workflow paths had no runtime coverage:

• The force dispatch input (bypasses the no-op promotion guard) had no e2e scenario.
• The rollback_on_failure path (rollback jobs that revert successful deploys when a sibling deploy fails) had no failure-injection scenario.

Changes

Harness (e2e/harness/):

• multistep.go: added RollbackOnFailure bool to PromoteStep with yaml tag rollback_on_failure. Mirrors the promote workflow's rollback_on_failure dispatch input.
• runner.go: wire RollbackOnFailure into executePromote - passes rollback_on_failure="true" in the workflow dispatch inputs when set.

Scenarios (e2e/scenarios/promote/):

• promote-force.yaml: three-step scenario. Clean promote succeeds; re-promote of the same SHA is rejected by the no-op guard (expect_failure); third promote with force: true bypasses the guard and succeeds. Asserts test state SHA matches dev after force.
• promote-rollback-runtime.yaml: commits two reusable deploy workflows (deploy-infra.yaml always succeeds; deploy-app.yaml checks out the promoted SHA and exit 1s on a marker file). First promote populates test state. A marker commit is orchestrated into dev. Second promote with rollback_on_failure: true runs both deploys - app fails on the marker, so rollback-infra runs to revert the successful infra deploy. Asserts preflight/promote/deploy-infra = success, deploy-app = failure, rollback-infra = success, rollback-app = skipped.

Verification

• go build ./... and cd e2e && go vet ./...: clean.
• Both scenarios pass under act/gitea locally (-timeout 9m each).
• Docker cleaned before and after each run.
• golangci-lint run ./e2e/...: clean.

Gitea-observable trim

The exact rollback_sha value passed to the infra deploy callback is trimmed to the job-conclusion claim (rollback-infra: success). On real GitHub the SHA is observable through deployment objects; gitea/act do not model those. The job-conclusion claim is the meaningful observable: the rollback path ran and completed successfully.