feat(Lifecycle): select plugin groups

[lengau]

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This adds the ability to choose plugin groups to the Lifecycle service.

This works differently from the default plugin setup in the Application,
which is now deprecated in favour of this. Apps that move over will be
rewarded with a happy fuzzy feeling and the ability to get the plugin
group based simply on the BuildInfo object.

Fixes #966 CRAFT-4933

[lengau]

We need a craft-parts release before merge.

[lengau]

This is just so that the pre-commit hooks stop complaining because the ======= looked like a merge conflict.

[Copilot]

Pull request overview

This PR introduces plugin group selection capability to the Lifecycle service, allowing applications to dynamically choose plugin sets based on BuildInfo. This deprecates the previous Application-based plugin registration approach in favor of a more flexible, build-specific plugin configuration system.

• Adds get_plugin_group() method to LifecycleService for dynamic plugin selection based on BuildInfo
• Implements witchcraft-specific plugin groups (MERLIN for Ubuntu ≤22.04 with Python, MORGANA for Ubuntu >22.04 with Rust)
• Deprecates Application.register_plugins() with a warning directing users to the new Lifecycle service approach

Reviewed changes

Copilot reviewed 14 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
witchcraft/services/lifecycle.py	New lifecycle service implementation with plugin group selection logic for witchcraft
witchcraft/services/init.py	Registers the custom lifecycle service with the ServiceFactory
craft_application/services/lifecycle.py	Adds base get_plugin_group() method and plugin group setup logic in LifecycleService
craft_application/application.py	Adds deprecation warning to register_plugins() method
tests/integration/services/test_lifecycle.py	Adds integration test for plugin group setup with different plugin configurations
tests/spread/witchcraft/plugin-groups/	Adds spread tests validating plugin group behavior on Ubuntu 22.04 and 24.04
pyproject.toml	Updates craft-parts dependency to git source (temporary development dependency)
uv.lock	Updates lockfile to reflect git-based craft-parts dependency
docs/reference/services/lifecycle.rst	New documentation page for LifecycleService
docs/reference/services/index.rst	Adds lifecycle service to documentation index
docs/reference/changelog.rst	Documents new feature and deprecation in 6.1.0 changelog

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[Copilot]

The summary incorrectly states "Ubuntu 22.04" when the base is "ubuntu@24.04". This should be corrected to "Ubuntu 24.04".

[Copilot]

The commented-out line with the pinned craft-parts version should be removed. Keeping commented-out dependencies in the file creates confusion and makes it unclear what the intended state is. If this git dependency is temporary for development, consider documenting that in a comment, or if it's permanent, remove the commented line.

[smethnani]

Small nit re the warning message but the rest looks good to me. Thanks @lengau!

[jahn-junior]

LGTM! Consider my suggestions optional :)