fix(vcs): address Phase 1 review findings

- Rename PullRequest.Head.Sha → SHA (Go acronym convention)
- Add typed ReviewEvent alias with exported constants
- Remove duplicate package doc from types.go (kept in interfaces.go)
- Fix misleading comment in check.go

vcs/types.go

@@ -1,14 +1,23 @@
-// 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
 
+// ReviewEvent is the event type for a pull request review action.
+type ReviewEvent string
+
+const (
+	// ReviewEventApprove approves the pull request.
+	ReviewEventApprove ReviewEvent = "APPROVED"
+	// ReviewEventRequestChanges requests changes to the pull request.
+	ReviewEventRequestChanges ReviewEvent = "REQUEST_CHANGES"
+	// ReviewEventComment posts a review comment without approval or rejection.
+	ReviewEventComment ReviewEvent = "COMMENT"
+)
+
 // PullRequest holds relevant PR metadata.
 type PullRequest struct {
 	Title string `json:"title"`
 	Body  string `json:"body"`
 	Head  struct {
-		Sha string `json:"sha"`
+		SHA string `json:"sha"`
 		Ref string `json:"ref"`
 	} `json:"head"`
 }
@@ -56,7 +65,7 @@ type ReviewComment struct {
 type ReviewRequest struct {
 	// Body is the top-level review comment.
 	Body string `json:"body"`
-	// Event is "APPROVED" or "REQUEST_CHANGES".
-	Event    string          `json:"event"`
+	// Event is the review action (approve, request changes, or comment).
+	Event    ReviewEvent     `json:"event"`
 	Comments []ReviewComment `json:"comments,omitempty"`
 }