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)

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
+}

vcs/types.go

@@ -0,0 +1,62 @@
+// Package vcs defines the shared interface and types for VCS platform clients.
+// Adapters (gitea, github) implement these interfaces; the core review logic
+// uses them without knowing the underlying platform.
+package vcs
+
+// PullRequest holds relevant PR metadata.
+type PullRequest struct {
+	Title string `json:"title"`
+	Body  string `json:"body"`
+	Head  struct {
+		Sha string `json:"sha"`
+		Ref string `json:"ref"`
+	} `json:"head"`
+}
+
+// ChangedFile represents a file modified in a PR.
+type ChangedFile struct {
+	Filename string `json:"filename"`
+	Status   string `json:"status"`
+}
+
+// ContentEntry represents a file or directory entry from the contents API.
+type ContentEntry struct {
+	Name string `json:"name"`
+	Path string `json:"path"`
+	Type string `json:"type"` // "file" or "dir"
+}
+
+// Review represents a pull request review.
+type Review struct {
+	ID   int64  `json:"id"`
+	Body string `json:"body"`
+	User struct {
+		Login string `json:"login"`
+	} `json:"user"`
+	State    string `json:"state"`
+	Stale    bool   `json:"stale"`
+	CommitID string `json:"commit_id"`
+}
+
+// ReviewComment represents an inline comment in a review.
+// All adapters use GitHub diff-position convention:
+//   - Position is a 1-indexed offset from the @@ hunk line in the unified diff.
+//   - CommitID is the commit SHA the comment is anchored to.
+//
+// Adapters are responsible for translating to/from platform-native formats
+// (e.g. Gitea uses line numbers; GitHub uses diff positions natively).
+type ReviewComment struct {
+	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"`
+}
+
+// ReviewRequest is the payload for posting a review.
+type ReviewRequest struct {
+	// Body is the top-level review comment.
+	Body string `json:"body"`
+	// Event is "APPROVED" or "REQUEST_CHANGES".
+	Event    string          `json:"event"`
+	Comments []ReviewComment `json:"comments,omitempty"`
+}