feat(vcs): extract interfaces and types from gitea/ (Phase 1, #78)

Add vcs/interfaces.go and vcs/types.go as the foundation for multi-platform
VCS support. Interfaces are discovered from working gitea/client.go code,
not invented in a vacuum.

vcs/interfaces.go — role-based interfaces:
- PRReader: GetPullRequest, GetPullRequestDiff, GetPullRequestFiles
- FileReader: GetFileContent (path + ref), ListContents
- Reviewer: PostReview (ReviewRequest), ListReviews, DeleteReview
- Identity: GetAuthenticatedUser
- Client: all four composed

vcs/types.go — types extracted from gitea/:
- PullRequest, ChangedFile, ContentEntry, Review (identical to gitea/)
- ReviewComment: uses GitHub diff-position convention (Position int,
  CommitID string) instead of Gitea's NewPosition int64
- ReviewRequest: new type wrapping Body, Event, Comments

vcs/check.go (//go:build ignore) — documents the gaps gitea.Client
must bridge in Phase 2:
1. PostReview signature mismatch (event+body+[]ReviewComment vs ReviewRequest)
2. GetFileContent missing ref parameter
3. ReviewComment type mismatch (NewPosition vs Position/CommitID)

No behavior changes. All existing tests pass.

vcs/check.go

@@ -0,0 +1,30 @@
+//go:build ignore
+
+// This file is excluded from normal builds.
+// Run `go build -v .` inside this file to see the documented gaps between
+// gitea.Client and vcs.Client that Phase 2 (Gitea adapter) must bridge.
+//
+// Known gaps (as of Phase 1):
+//
+//  1. PostReview signature mismatch:
+//     gitea.Client:  PostReview(ctx, owner, repo, number, event, body string, comments []gitea.ReviewComment)
+//     vcs.Reviewer:  PostReview(ctx, owner, repo, number, req vcs.ReviewRequest)
+//
+//  2. GetFileContent signature mismatch:
+//     gitea.Client:  GetFileContent(ctx, owner, repo, filepath string)  [no ref; uses default branch]
+//     vcs.FileReader: GetFileContent(ctx, owner, repo, path, ref string)
+//     (gitea.Client has GetFileContentRef for the ref variant)
+//
+//  3. ReviewComment type mismatch:
+//     gitea.ReviewComment uses NewPosition int64 (Gitea line-number convention)
+//     vcs.ReviewComment uses Position int (GitHub diff-position convention)
+//
+// The Gitea adapter (Phase 2) will wrap gitea.Client to bridge these gaps.
+
+package vcs
+
+import "gitea.weiker.me/rodin/review-bot/gitea"
+
+// This compile-time assertion intentionally fails.
+// It documents what the Gitea adapter must implement to satisfy vcs.Client.
+var _ Client = (*gitea.Client)(nil)