test: run real external-update and notify path in multi-repo e2e

[joshua-temple]

Problem

The multi-repo e2e harness was simulation-dominant. The generated external-update.yaml,
the cascade external update verb, and the satellite notify step were never executed:
cross-repo dispatch recorded external state straight into an in-memory ExecutionContext
and the final assertions read it back from the same place, so the external/notify product
surface had no real coverage.

Fix

• Generate real workflows per repo. MultiRepoHarness.CreateRepo now writes the manifest to
  .github/manifest.yaml (the CLI default; it previously wrote a path the CLI never reads)
  carrying both config and state, then runs cascade generate-workflow for each repo,
  reusing the single-repo generate/localize/converge machinery.
• Drive the real verb. Cross-repo dispatch now runs the primary's external-update.yaml
  under act with the inputs the satellite would send. The real cascade external update
  verb writes state.<env>.external.<name> back to the primary's manifest in gitea, and the
  final assertions read that committed manifest instead of in-memory state.
• Assert the real notify step content. For each notifying satellite, the generated
  orchestrate.yaml is read from gitea and checked for the dispatch step targeting the
  primary's external-update workflow.
• Product fix: the generated external-update.yaml ran the update verb (which commits and
  pushes the manifest) without first setting a git identity, which aborts on a clean runner.
  It now emits a Configure Git step before the verb, matching the other state-writing
  workflows.

The live cross-repo dispatch network hop is a no-op against gitea (it does not implement the
Actions dispatch API), so the harness bridges it by invoking the primary's external-update
workflow directly with the same inputs. The network hop itself is real-GitHub validation
material and is documented as residue in the harness.

Verification

• go build ./..., go vet ./e2e/..., golangci-lint run ./e2e/... and
  ./internal/generate/... all clean.
• Three multi-repo scenarios pass under act with real manifest-read assertions:
  satellite-notifies-primary, external-state-promotion, mixed-deploys. The rollback scenario
  still passes against the real manifest. Product generate tests and harness unit tests green.