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/interfaces.go

@@ -0,0 +1,37 @@
+package vcs
+
+import "context"
+
+// PRReader can fetch pull request metadata, diffs, and changed files.
+type PRReader interface {
+	GetPullRequest(ctx context.Context, owner, repo string, number int) (*PullRequest, error)
+	GetPullRequestDiff(ctx context.Context, owner, repo string, number int) (string, error)
+	GetPullRequestFiles(ctx context.Context, owner, repo string, number int) ([]ChangedFile, error)
+}
+
+// FileReader can fetch file contents and list directory entries.
+type FileReader interface {
+	GetFileContent(ctx context.Context, owner, repo, path, ref string) (string, error)
+	ListContents(ctx context.Context, owner, repo, path string) ([]ContentEntry, error)
+}
+
+// Reviewer can post, list, and delete pull request reviews.
+type Reviewer interface {
+	PostReview(ctx context.Context, owner, repo string, number int, req ReviewRequest) (*Review, error)
+	ListReviews(ctx context.Context, owner, repo string, number int) ([]Review, error)
+	DeleteReview(ctx context.Context, owner, repo string, number int, reviewID int64) error
+}
+
+// Identity can report who the authenticated user is.
+type Identity interface {
+	GetAuthenticatedUser(ctx context.Context) (string, error)
+}
+
+// Client is the full VCS interface: PR reads, file reads, review management, and identity.
+// Platform adapters (gitea, github) implement this interface.
+type Client interface {
+	PRReader
+	FileReader
+	Reviewer
+	Identity
+}