feat(ci): install packaged GitHub Actions workflows in customer projects

## Problem

Customers need an easy starting point for running kongctl in CI/CD. Existing
examples and guidance still leave users assembling their own workflow. The
immediate user request is simple: install kongctl in CI and run a plan.

## Proposal

Add a command that installs packaged GitHub Actions workflows into a customer
project, starting with one basic plan workflow. For example:

```sh
kongctl install workflows
```

The command name is provisional. Align it with the broader generation proposal
in #486 during implementation; this issue tracks a small initial deliverable
from that broader idea.

Install an editable workflow file under the project's `.github/workflows/`
directory. Package the initial template with kongctl and leave room to add more
workflow templates later.

## Initial scope

- Provide one reference workflow that checks out the customer repository,
installs kongctl, authenticates to Konnect using a GitHub Actions secret,
and runs `kongctl plan` against the project's declarative configuration.
- Use pull requests as the initial CI trigger, with manual execution available
for getting started and verification.
- Let users specify the declarative configuration path and document the
required secret and any other setup. Make the kongctl version used by the
workflow explicit and easy to change.
- Write the workflow locally so users can review, customize, and commit it.
Report the generated path and remaining setup steps.
- Preserve existing files: repeated installation must not silently overwrite
a customer's workflow.

## Acceptance criteria

- A user can install the packaged workflow into a local project with a single
command, then configure the documented secret and configuration path.
- The generated workflow installs kongctl and successfully runs a plan for a
valid configuration; installation or planning failures fail the CI job.
- Plan output is available in the workflow run for inspection.
- The initial workflow does not execute `apply` or `sync`.
- Documentation includes a minimal end-to-end getting-started example.
- Tests cover command behavior, workflow generation, and existing-file handling;
the generated workflow is validated.

## Follow-up scope

Additional templates can address plan review and approval, apply on merge,
sync versus apply, multiple environments, and single-repo, multi-repo, or
federated setups. Those choices should not expand this first iteration into a
universal CI/CD solution.

Related: #486 (broader declarative configuration and workflow generation).

Start at the CLI command entry point and the project’s workflow packaging and generation paths; compare the proposed command with the broader generation work in #486. Add coverage for command behavior, workflow validation, generation, and existing-file handling, then verify that the documented end-to-end setup produces a failing-on-error plan workflow without apply or sync.

Written by the indexing model from the issue text.