Remove retired ChromeOS and Tast integration from kernelci-core

## Context

KernelCI no longer intends to maintain ChromeOS-specific build and test coverage in `kernelci-core`.

The legacy ChromiumOS rootfs builder was removed in commit [`b1e2f162`](https://github.com/kernelci/kernelci-core/commit/b1e2f1624c78282354568402b7080b9dd77ec276), but a substantial part of the integration remains. This includes ChromeOS configuration downloads in the kernel builders, Tast and ChromeOS LAVA templates, dedicated Docker images, Debos rootfs variants, parsers, flashing helpers, and compatibility mappings.

These remaining components should be retired instead of updating the hard-coded ChromeOS LTS fallback or continuing to maintain unused external dependencies.

## Goal

Remove the dedicated ChromeOS and Tast integration from `kernelci-core` while preserving generic Linux support for Chromebook hardware where it is still used by normal KernelCI jobs.

The cleanup should leave no active configuration capable of scheduling a ChromeOS/Tast job or building a ChromeOS-specific image.

## Repository cleanup

### Kernel build support

- Remove the `cros://` configuration-fragment implementation from `kernelci/kbuild.py`, including:
- `CROS_CONFIG_URL`;
- `LATEST_LTS_MAJOR` and `LATEST_LTS_MINOR`;
- `_getcrosfragment()`;
- the ChromeOS-specific defconfig and fragment branches.
- Remove the corresponding `cros://` implementation from `kernelci/build.py`.
- Remove `crosarch` and `CROS_ARCH` formatting if no non-ChromeOS consumer remains.
- Remove the unused `krev` formatting argument if no non-ChromeOS configuration uses it.
- Remove ChromeOS-only shortening rules from `kernelci/test.py`.

### Runtime and result handling

- Remove `config/runtime/chromeos/base.jinja2`.
- Remove the dedicated test templates:
- `config/runtime/tests/tast.jinja2`;
- `config/runtime/tests/tast-debian.jinja2`;
- `config/runtime/tests/fluster-chromeos.jinja2`.
- Remove Tast-only result parsing from `kernelci/runtime/lava.py`.
- Replace ChromeOS/Tast-specific examples in otherwise generic API model descriptions.
- Keep `config/runtime/tests/blktests-ddp.jinja2`, but remove its copied `/home/cros/lava` path and `cros-tast` metadata if the template is still active.

### Docker definitions and helper data

Remove the dedicated or orphaned ChromeOS Docker definitions:

- `config/docker/cros-baseline.jinja2`;
- `config/docker/cros-qemu-modules.jinja2`;
- `config/docker/cros-sdk.jinja2`;
- `config/docker/cros-tast.jinja2`;
- `config/docker/fragment/cros-lava.jinja2`;
- `config/docker/fragment/gsutil.jinja2`, if it has no remaining consumer.

Remove helper data used only by those images or templates, including:

- `config/docker/data/add_modules.sh`;
- `config/docker/data/tast_parser.py`;
- `config/docker/data/gcov_pack.sh`;
- `config/docker/data/gcov_reset.sh`;
- `config/docker/data/ssh_retry.sh`.

Before deleting a helper with a generic filename, verify with a repository-wide search that it has no non-ChromeOS consumer.

### Rootfs definitions, scripts, and overlays

- Remove the `trixie-tast` rootfs definition.
- Remove the `trixie-cros-ec` rootfs definition and `trixie-cros-ec-tests.sh` unless a maintainer explicitly identifies an active, non-ChromeOS user that should be split into a separately named rootfs.
- Remove the Tast build script and overlay:
- `config/rootfs/debos/scripts/trixie-tast.sh`;
- `config/rootfs/debos/overlays/tast/`.
- Remove the orphaned ChromeOS flashing support:
- `config/rootfs/debos/scripts/trixie-cros-flash.sh`;
- `config/rootfs/debos/overlays/cros-flash/`.
- Review the legacy/generated `bookworm-cros-ec` rootfs entry. Do not patch `config/core/rootfs-images.yaml` as an ordinary source file; use the maintainer-approved legacy retirement or regeneration procedure.

### Documentation and tests

- Remove or update obsolete ChromeOS/Tast examples in maintained documentation and test fixtures.
- Add regression checks that reject references to removed rootfs names, templates, Docker definitions, and `cros://` fragments.
- Keep ordinary Linux kernel configuration symbols such as `CONFIG_CROS_EC` and `CONFIG_CHROMEOS_LAPTOP` when they are required to test Chromebook hardware with mainline Linux.

## Cross-repository and operational cleanup

Repository automation cannot determine whether external production systems still reference these names. Before merging the final removal:

1. A KernelCI maintainer checks active configuration in `kernelci-pipeline`, deployment repositories, scheduled workflows, and lab configuration for references to the removed templates, rootfs variants, and Docker images.
2. A lab or operations owner confirms that no ChromeOS/Tast jobs are still scheduled.
3. A maintainer decides whether published ChromeOS rootfs artifacts and container images should be retained as historical artifacts, archived, or deleted. External artifacts must not be deleted solely as part of the code cleanup PR.
4. Any scheduled image builds or publication jobs are disabled before their source definitions are removed.

Record the outcome of these checks in this issue.

## Existing issues to triage after retirement

The following open issues appear to become obsolete when ChromeOS testing is retired and should be reviewed, commented on, and closed as appropriate:

- #1691
- #1692
- #1760
- #1917
- #1960
- #2181
- #2813

#1690 concerns physical Chromebook hardware and should be reviewed separately because the devices may still be useful for non-ChromeOS mainline Linux testing.

Generic work such as #2464 should remain open if it still applies to non-ChromeOS operating systems.

After the issue backlog is cleaned up, maintainers should decide whether the `chromeos`, `chromeos roadmap`, and `staging:chromeos` labels are still needed.

## Acceptance criteria

- No active KernelCI configuration builds or schedules ChromeOS or Tast jobs.
- `cros://` build-fragment support and the hard-coded latest-LTS fallback are removed.
- Dedicated ChromeOS/Tast runtime templates, Docker definitions, rootfs definitions, scripts, overlays, and parsers are removed.
- Shared files are deleted only after confirming that they have no non-ChromeOS consumers.
- Generic mainline-Linux support for Chromebook hardware is either retained or its removal is explicitly approved by the relevant hardware owner.
- Maintainers confirm that deployment and pipeline repositories no longer reference the removed components.
- The disposition of previously published rootfs and container images is documented.
- Obsolete ChromeOS issues are commented on and closed after the retirement lands.
- Configuration validation, template tests, and the full repository test suite pass.

## Out of scope

- Removing generic Linux drivers or kernel configuration symbols for Chromebook hardware merely because their names contain `CROS` or `CHROMEOS`.
- Decommissioning physical lab devices that can still run non-ChromeOS KernelCI tests.
- Deleting published artifacts or container images without an explicit operations-owner decision.
- Replacing ChromeOS/Tast coverage with a different test system as part of the same change.

Start with repository-wide searches for cros://, Tast, the named runtime and Docker templates, rootfs definitions, and helper scripts, then inspect kernelci/kbuild.py, kernelci/build.py, kernelci/runtime/lava.py, and the referenced configuration directories. Confirm non-ChromeOS consumers and external pipeline references with maintainers before removing shared files; done means the listed components are retired, generic Chromebook Linux support remains, validation and tests pass, and operational checks are recorded.

Written by the indexing model from the issue text.