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.

@@ -0,0 +1,30 @@
+//go:build ignore
+
+// This file is excluded from normal builds.

@@ -0,0 +20,4 @@
+
+// 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)

@@ -0,0 +4,4 @@
+package vcs
+
+// PullRequest holds relevant PR metadata.
+type PullRequest struct {

@@ -0,0 +49,4 @@
+	Path     string `json:"path"`
+	Position int    `json:"position"` // diff-position: 1-indexed offset from @@ hunk line
+	CommitID string `json:"commit_id"`
+	Body     string `json:"body"`

@@ -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

@@ -0,0 +7,4 @@
+type PullRequest struct {
+	Title string `json:"title"`
+	Body  string `json:"body"`
+	Head  struct {

@@ -0,0 +55,4 @@
+// ReviewRequest is the payload for posting a review.
+type ReviewRequest struct {
+	// Body is the top-level review comment.
+	Body string `json:"body"`

@@ -0,0 +27,4 @@
+
+// This compile-time assertion intentionally fails.
+// It documents what the Gitea adapter must implement to satisfy vcs.Client.
+var _ Client = (*gitea.Client)(nil)

@@ -0,0 +14,4 @@
+
+// PullRequest holds relevant PR metadata.
+type PullRequest struct {
+	Title string `json:"title"`

@@ -0,0 +42,4 @@
+	User struct {
+		Login string `json:"login"`
+	} `json:"user"`
+	State    string `json:"state"`

@@ -0,0 +1,30 @@
+//go:build ignore

@@ -0,0 +5,4 @@
+
+const (
+	// ReviewEventApprove approves the pull request.
+	ReviewEventApprove ReviewEvent = "APPROVED"

@@ -0,0 +24,4 @@
+//     vcs.ReviewComment uses Position int (GitHub diff-position convention)
+//
+// The Gitea adapter (Phase 2) will wrap gitea.Client to bridge these gaps.
+var _ vcs.Client = (*gitea.Client)(nil)

@@ -0,0 +1,82 @@
+package vcs

@@ -0,0 +55,4 @@
+	Stale    bool     `json:"stale"`
+	CommitID string   `json:"commit_id"`
+}
+