feat(hooks): invoke customActions from deploy.*.hooks.before/after

[bogdannazarenko]

Phase 3: Cloud Deploy custom-target parity — action hooks

Final phase of the three-part series started in #10065 and continued in #10066. Neither earlier PR is required to land first; this branch is cut from main.

What's in

• New action: hook union member on DeployHookItem:

  deploy:
    kubectl:
      hooks:
        before:
          - action: { name: db-migrate }
        after:
          - action: { name: smoke-test }
  customActions:
    - name: db-migrate
      containers: [{ image: myorg/migrator, ... }]

  Reuses the existing customActions runtime — a single action definition is runnable standalone via skaffold exec or wrapped as a deploy hook, so users aren't forced to choose between the two shapes.

• ActionInvoker interface in pkg/skaffold/hooks. A tiny SetDefaultActionInvoker setter is wired once in runner.New right after GetActionsRunner succeeds. The hooks package stays free of any pkg/skaffold/actions import, preserving the existing dependency tree.

• actionsRunnerInvoker adapter in pkg/skaffold/runner satisfies hooks.ActionInvoker by delegating to the runner's Exec with nil artifact slices — hooks are not themselves build producers.

• Load-time validation (validateActionHookRefs): unknown action references fail at config parse rather than deep inside a deploy. Missing name fields are flagged too. Covers kubectl, helm, and kpt deployers.

• Unit tests (pkg/skaffold/hooks/action_test.go): ordered dispatch, first-failure aborts remaining hooks, missing-invoker sentinel.

• Docs + runnable example: new 'Action hooks' section in docs-v2/docs/lifecycle-hooks.md; examples/hooks-action/ (mirrored under integration/examples/) exercises the schema end-to-end with pre-deploy + post-deploy busybox actions.

What's deliberately out of scope

• CloudRunDeployHooks still accepts []HostHook only. Wiring action: support into the CloudRun deployer would require promoting its hook list to a union type; saving that for a dedicated follow-up to keep this diff focused.
• Pre-delete / post-delete hooks (PLAN's Phase 4).
• Render / build ActionHook support — not requested.
• No schema version bump: this PR keeps latest at skaffold/v4beta14 per the maintainer-driven hack/new-version.sh workflow. If you'd prefer the freeze done here, happy to add a commit.

Test results

• go build ./... — clean
• go test -short -race ./pkg/skaffold/hooks/... ./pkg/skaffold/schema/validation/... ./pkg/skaffold/runner/... ./pkg/skaffold/deploy/... ./cmd/... — green
  (the pre-existing TestNewEnvClient/DOCKER_HOST_not_set,_output_invalid_host and TestNewRunner failures reproduce on main and are unrelated)
• ./hack/check-samples.sh — passes

[gemini-code-assist]

Code Review

This pull request introduces 'Action hooks,' allowing users to reference custom actions as deployment lifecycle hooks. The implementation includes the ActionInvoker interface to prevent package cycles, updated validation to ensure action names exist, and new documentation and examples. A review comment points out that adding fields to the v4beta14 schema might violate versioning policies if that version is already frozen, suggesting a move to a new schema version.

[bogdannazarenko]

Waiting on maintainers to cut new schema version on main branch