Surface deploy parameters (--set / --set-value-file) to exec custom-action and verify containers for Cloud Deploy parity

## Problem

[Google Cloud Deploy passes deploy parameters](https://docs.cloud.google.com/deploy/docs/parameters) to its execution environment as environment variables, surfacing them to render, **custom action**, and **verify** containers. Skaffold already accepts deploy parameters via `--set` / `--set-value-file`, but historically they were only used for **manifest templating** and only exposed on a subset of commands (`render`, `filter`, `delete`). As a result:

- `skaffold exec` (custom actions) did **not** inject deploy parameters as environment variables into action containers.
- `skaffold verify` did **not** accept `--set` / `--set-value-file` at all, and never injected deploy parameters into verify containers.

This breaks Cloud Deploy parity. A custom action (e.g. an Alembic database migration) cannot read a Cloud SQL connection URL passed as a deploy parameter, and a post-deploy `verify` smoke test cannot read the deployed Cloud Run service URL that Cloud Deploy injects automatically.

## Expected behavior

`--set` / `--set-value-file` should be available on `deploy`, `dev`, `run`, `exec`, and `verify` (in addition to the existing `render` / `filter` / `delete`). On `exec` and `verify`, the supplied key/value pairs should additionally be injected as environment variables into every custom-action / verify container, with precedence (lowest → highest):

1. `--env-file` (base)
2. `--set-value-file`
3. `--set`

## Current behavior

- `skaffold verify` rejects `--set` / `--set-value-file` (flags are not registered on the command).
- `exec` / `verify` containers receive no deploy parameters as environment variables.

## Reproduction

**Verify (Cloud Run service URL → Playwright smoke test):**

```yaml
# skaffold.yaml
apiVersion: skaffold/v4beta14
kind: Config
metadata:
name: repro
verify:
- name: smoke-test
container:
name: smoke
image: alpine:3.15.4
command: ["/bin/sh", "-c"]
args: ["echo \"smoke test against ${SERVICE_URL}\""]
```

```console
$ skaffold verify --set SERVICE_URL=https://my-service-abc-uc.a.run.app
Error: unknown flag: --set # today
# expected: the container prints "smoke test against https://my-service-abc-uc.a.run.app"
```

**Exec (Cloud SQL URL → Alembic migration):**

```yaml
# skaffold.yaml
apiVersion: skaffold/v4beta14
kind: Config
metadata:
name: repro
customActions:
- name: db-migrate
containers:
- name: alembic
image: alpine:3.15.4
command: ["/bin/sh", "-c"]
args: ["echo \"alembic upgrade head -> ${DATABASE_URL}\""]
```

```console
$ skaffold exec db-migrate --set DATABASE_URL=postgresql+pg8000://user:pass@/app?unix_sock=/cloudsql/PROJECT:REGION:INSTANCE/.s.PGSQL.5432
# today: DATABASE_URL is empty inside the container
# expected: the container prints the full connection URL
```

## Proposed solution / scope

- Expose `--set` / `--set-value-file` on `deploy`, `dev`, `run`, `exec`, and `verify`.
- Inject the merged deploy parameters as environment variables into custom-action containers (`exec`) and verify containers (`verify`), reusing one shared merge helper with the precedence above.
- Add runnable examples (custom-action Cloud SQL/Alembic migration; Playwright `verify` smoke test) and document the behavior on the custom-actions and verify pages.

Start by tracing flag registration for deploy, dev, run, exec, and verify, then follow how environment variables are assembled for custom-action and verify containers. Compare the existing --set and --set-value-file behavior with --env-file precedence; done means both commands accept the flags, containers receive the merged values, and the requested runnable examples and documentation are included.

Written by the indexing model from the issue text.