feat(e2e): plumb act event payload and env/* branch fetch

[joshua-temple]

Problem

Two harness gaps blocked the hotfix e2e flow:

1. RunOpts.EventJSON was declared in act.go but never plumbed into the act invocation - the event payload was silently dropped on every run.
2. SyncRepoToActContainer only fetched origin main; act jobs that inspect env/* branches or tags (plan/apply/finalize) could not see them.

Changes

• act.go: When RunOpts.EventJSON is set, write the payload to /tmp/cascade-event.json inside the act container via CopyToContainer, then pass -e /tmp/cascade-event.json to the act invocation. Applies to both act run paths (RunWorkflow and RunWorkflowFromRepo). Flag construction is factored into a pure eventFileArgs helper for unit testing.
• harness.go (SyncRepoToActContainer): after the load-bearing git fetch origin main && git reset --hard origin/main, also fetch +refs/heads/env/*:refs/remotes/origin/env/* and --tags, mirroring the fetch step in the generated hotfix workflow. The extra fetch is non-fatal (|| true) so the bounded-retry and lost-commit guarantees are preserved.
• harness.go (localizeWorkflows): confirmed the existing yaml glob covers cascade-hotfix.yaml; added a comment noting the workflow name: field is only suffixed (not rewritten), preserving future workflow_files assertions over hotfix job/trigger content.
• act_test.go: table-driven unit tests verifying -e <file> appears in the act command when an event payload is written and is omitted otherwise.

Apply-job strategy (recorded, not implemented here)

The generated apply job shells out to gh pr create/gh pr merge --auto, which does not speak Gitea's API. Therefore the apply job is not run under act. A later unit will drive the cherry-pick/PR/merge via harness primitives (real git cherry-pick + Gitea PRs), and then fire the merged-PR event payload (assembled by this EventJSON plumbing) so act runs the context/build/deploy/finalize jobs for real.

Verification

• go build ./... and go vet ./... pass for both root and e2e modules.
• TestMultiStepScenarios/Two_Environment and Three_Environment pass (no regression).
• New eventFileArgs/buildActArgs unit tests pass.
• golangci-lint run ./e2e/... clean.