Consolidate VCS integration orchestration

[khvn26]

The same logical events (link, unlink, feature state changed, feature deleted) dispatch per-provider from multiple unrelated sites today. Adding a new VCS (Bitbucket, say) means touching all of these files and re-learning the conventions each time.

Current state

After #7306 and #7327, integrations/vcs/services.py hosts:

• dispatch_vcs_on_resource_create(resource)
• dispatch_vcs_on_resource_destroy(resource)

FeatureExternalResourceViewSet.perform_create and .perform_destroy delegate to these. GitLab side effects (log, webhook register/deregister, initial tag, tag clear, link/unlink comment) all flow through the dispatcher via namespaced calls (from integrations.gitlab import services as gitlab), and the GitLab-specific apply_gitlab_tag / deregister_gitlab_webhook lifecycle hooks on FeatureExternalResource are gone.

Still to do

View-level, outside the dispatcher:

• FeatureExternalResourceViewSet.create still owns the GitHub URL preflight regex and inline label_github_issue_pr call.

Serializer-level:

• State change: FeatureStateSerializerBasic.save calls call_github_task (GitHub) and post_gitlab_state_change_comment_for_feature_state (GitLab) directly. No dispatch_vcs_on_feature_state_changed yet.
• features/versioning/serializers.py — same split for the v2 versioning flow.

Model-level hooks still present — consolidate to single VCS hook per model:

• FeatureExternalResource.notify_github_on_link (AFTER_SAVE, type is GITHUB_ISSUE/GITHUB_PR) — applies GitHub tag + link comment.
• FeatureExternalResource.notify_github_on_unlink (BEFORE_DELETE, type is GITHUB_ISSUE/GITHUB_PR) — unlink comment to GitHub.
• Feature.create_github_comment (AFTER_SAVE) — flag-deleted comment to GitHub.
• Feature.create_gitlab_comment (AFTER_SAVE) — flag-deleted comment to GitLab.

Proposed shape for the rest

Extend integrations/vcs/services.py with:

• dispatch_vcs_on_feature_state_changed(feature_state)
• dispatch_vcs_on_feature_deleted(feature_name, feature_id, project_id)

Port each remaining trigger behind these:

• Move the four model-level hooks above into the dispatcher's create/destroy paths (for the FER hooks) and the feature-deleted dispatcher (for the Feature hooks). Model-level triggers should be a last resort; when a side effect is driven by a viewset action, the dispatcher called from the viewset is the honest place for it.
• Move the two serializer dispatch sites behind dispatch_vcs_on_feature_state_changed.
• Decide whether the GitHub URL preflight regex stays in the view or moves into a provider-owned validator hook called from dispatch_vcs_on_resource_create.

The current implementation uses explicit if resource.type in GITLAB_RESOURCE_TYPES: branches rather than a registry protocol (AppConfig.ready()). Simple, fine for two providers; worth revisiting if a third joins.

Not in scope

• Emit-parity between GitHub and GitLab (state-change, delete comments for GitHub). Separate product call.
• Log-event consolidation — falls out of the dispatch model naturally.