diagnose: --enable-templating defaults to false, breaking Cloud Deploy deploy-parameter interpolation

## Description

Google Cloud Deploy's render phase generates the canonical `config.yaml` for a release internally via `skaffold diagnose`, but invokes it without `--enable-templating=true`. As a result, any Skaffold config field that supports Go-template interpolation (tagged `skaffold:"...,template"`, e.g. `deploy.kubectl.defaultNamespace`) is left as a literal, unexpanded template string instead of being populated from the deploy parameters / environment variables that Cloud Deploy injects for the release.

This further cascades into downstream steps — e.g. the `verify` job — that rely on the resolved `deploy.kubectl.defaultNamespace`.

## Relevant log (Cloud Deploy render phase, trimmed to the relevant `skaffold` invocations)

```
Running the following command: skaffold [render --filename=/workspace/stable/config.yaml --build-artifacts=/workspace/artifacts.json --output=/workspace/stable/manifest.yaml --offline=true --digest-source=none --set="environment=pr-88" --set="git_sha="]
--digest-source set to 'none', tags listed in Kubernetes manifests will be used for render
Running the following command: skaffold [inspect namespaces list /workspace/stable/manifest.yaml --filename=/workspace/stable/config.yaml]
{"resourceToInfoMap":{"apps/v1, Kind=Deployment":[{"name":"myapp-gke","namespace":"myapp-gke-{{.environment}}"}]}}
```

Note `"namespace":"myapp-gke-{{.environment}}"` — the `environment` deploy parameter (`pr-88` in this release) was never substituted into the template, because `/workspace/stable/config.yaml` (the effective config Cloud Deploy generated via `skaffold diagnose`) was produced without `--enable-templating`.

## Root cause

`skaffold diagnose --enable-templating` defaults to `false` (`cmd/skaffold/app/cmd/diagnose.go`). Cloud Deploy never passes `--enable-templating=true` when it calls `diagnose` to produce its internal `config.yaml`, so fields tagged `skaffold:"...,template"` are never expanded.

## Reproduction

Minimal `skaffold.yaml` that templates `deploy.kubectl.defaultNamespace` from a deploy parameter named `environment`:

```yaml
apiVersion: skaffold/v4beta14
kind: Config
metadata:
name: myapp
build:
artifacts:
- image: myapp
docker:
dockerfile: Dockerfile
manifests:
rawYaml:
- k8s/*.yaml
deploy:
kubectl:
defaultNamespace: "myapp-{{.environment}}"
```

```shell
# simulates how Cloud Deploy invokes diagnose internally (deploy parameter as env var, no --enable-templating)
$ environment=pr-88 skaffold diagnose --yaml-only -f skaffold.yaml
...
deploy:
kubectl:
defaultNamespace: myapp-{{.environment}} # BUG: not interpolated

# manual workaround, passing the flag explicitly
$ environment=pr-88 skaffold diagnose --yaml-only -f skaffold.yaml --enable-templating
...
deploy:
kubectl:
defaultNamespace: myapp-pr-88 # expected
```

## Expected behavior

Deploy parameters supplied as environment variables (as Google Cloud Deploy does) should be interpolated into templated fields by default, without requiring Cloud Deploy (or any other caller of `diagnose`) to explicitly pass `--enable-templating=true`.

## Proposed fix

Default `--enable-templating` to `true` on `skaffold diagnose`. Templating is opt-out (`--enable-templating=false`) rather than opt-in, and `expandTemplate` already leaves a field untouched if expansion produces `` (e.g. when the underlying env var isn't set), so this does not change behavior for configs that aren't using templated fields.

Start in cmd/skaffold/app/cmd/diagnose.go and inspect how the --enable-templating default is defined. Reproduce the behavior with the provided skaffold diagnose commands and an environment-backed template, then verify that templated fields expand by default while explicitly disabling templating preserves the opt-out behavior.

Written by the indexing model from the issue text.