feat(integrations): add merge-method selection and provider-specific payloads

Introduce dedicated merge handling for integration review merges:

- Add src-tauri/src/integrations/merge.rs: implements merge_options (read provider repo settings), merge_payload (build provider-specific merge body) and a Tauri command get_integration_review_merge_options. Includes unit tests for behavior.
- Wire merge module into integrations.rs and pass an optional merge_method into provider-specific review action functions (GitHub, GitLab, Gitea, Azure DevOps). run_integration_review_action now accepts an optional merge_method, validates it early, and includes provider-specific merge payloads when performing a merge.
- Export the new command in src-tauri/src/main.rs so the frontend can request merge options.

Frontend changes to support selecting a merge method before merging:

- ConfirmDialog.svelte: add SelectMenu support and a select field to confirm requests.
- ReviewCenter.svelte: fetch integration merge options, show a merge-method selector in the merge confirmation, and pass the chosen method to the review action.
- Update types and git bindings to surface IntegrationMergeOptions / IntegrationMergeMethod and the getIntegrationReviewMergeOptions call (git.ts / types.ts changes staged).

Effect: users can pick a merge method appropriate to the provider/project; the integration layer generates the correct API payload per provider. Tests added for merge logic.

src/lib/types.ts

@@ -66,6 +66,9 @@ export interface GitIntegrationRepository {
 }
 
 export type IntegrationReviewState = "open" | "draft" | "merged" | "closed";
+export type IntegrationMergeMethod = "default" | "merge" | "squash" | "rebase" | "rebase-merge" | "fast-forward-only";
+export interface IntegrationMergeOptions { methods: IntegrationMergeMethod[]; defaultMethod: IntegrationMergeMethod; }
+
 export type IntegrationReviewAction = "merge" | "approve" | "close" | "reopen";
 
 export interface IntegrationReviewComment {