feat: generalize skill sync via GitRepoAdapter, add GitLab support

[renefichtmueller]

Summary

Addresses #13990 (generalize skill sync beyond GitHub) — scoped to GitLab only for this PR; Bitbucket and Azure DevOps are natural follow-ups once this pattern is proven, not included here.

The existing GitHub-only skill-sync runner (packages/api/src/skills/sync/github.ts) tightly interleaved GitHub-specific REST calls with generic orchestration logic (skill discovery, DB upsert/cleanup, sync counters, stale-delete ordering). This PR:

• Extracts a GitRepoAdapter interface (packages/api/src/skills/sync/adapters/types.ts) covering exactly the REST-specific surface the orchestration needs: resolveCommit, fetchTreeEntries, fetchFileContent.
• Moves the GitHub REST implementation into adapters/github.ts implementing that interface; github.ts itself now only contains the (unchanged) generic orchestration, calling through the adapter.
• Implements adapters/gitlab.ts (list tree via GET /projects/{id}/repository/tree?recursive=true, fetch blob via GET /projects/{id}/repository/files/{path}/raw, Private-Token auth, configurable baseUrl for self-hosted GitLab) and a parallel packages/api/src/skills/sync/gitlab.ts orchestrator mirroring github.ts.
• Adds a createRepoAdapter factory (adapters/factory.ts) that picks the right adapter by provider.
• Extends skillSyncConfigSchema (packages/data-provider/src/config.ts) with a gitlab key mirroring the github key's shape/validation, and extends SkillSyncProvider to 'github' | 'gitlab'.
• Adds 'gitlab' to the Skill.source enum and SkillSyncStatusInput/ISkillSyncStatus gain baseUrl/projectId fields alongside the existing GitHub owner/repo.

Regression safety was the top priority for the extraction. Every existing GitHub sync test passes unmodified in behavior — only import paths changed where the code moved into adapters/github.ts.

Test plan

• packages/api: npx jest src/skills/sync — 69/69 passing (includes all pre-existing GitHub orchestrator/scheduler tests unmodified, plus new adapters/gitlab.spec.ts and adapters/factory.spec.ts)
• packages/api: npx jest src/admin/skills.spec.ts — 10/10 passing (admin skill-sync API surface, unaffected)
• packages/data-schemas: npx jest src/methods/skillSync.spec.ts src/methods/skill.spec.ts — 106/106 passing
• packages/data-provider: npx jest src/config — 119/119 passing (schema extension didn't break existing config validation)
• tsc --noEmit clean on packages/api, packages/data-schemas, packages/data-provider
• eslint clean on all changed/new files