Add github_continuous_import feature flag

What does this MR do and why?

This MR adds a feature flag definition for the github_continuous_import feature, which will keep a GitLab project synced with its GitHub source after import. The feature is being ported from a proof-of-concept merge request (!251883, which will be closed, not merged) into master as a series of small, independently-reviewed merge requests. This flag needs to exist first so every later MR has something to gate its unfinished code behind, keeping it invisible until the feature is complete.

The flag is type: wip and disabled by default (default_enabled: false).

This MR also adds a single no-op Feature.enabled?(:github_continuous_import, project) guard clause in Stage::FinishImportWorker#import, so the rspec:feature-flags CI job (which fails on unused flags) passes. It has no effect today; the real continuous-sync behavior lands in a follow-up MR.

References

• #628579 (closed)
• gitlab-org#23553

Screenshots or screen recordings

Not applicable — this MR only adds a feature flag definition file, no UI change.

How to set up and validate locally

1. Verify config/feature_flags/wip/github_continuous_import.yml exists with type: wip and default_enabled: false.
2. In a Rails console, confirm Feature.enabled?(:github_continuous_import) returns false.
3. Confirm Stage::FinishImportWorker#import still marks imports as finished with the flag disabled (see the added spec context).

MR acceptance checklist

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