Add CommitID string field to vcs.ReviewRequest so both platform
adapters can thread the commit anchor through the abstraction layer.

Changes:
- vcs/types.go: Add CommitID field with json tag commit_id,omitempty
- gitea/client.go: Re-add commitID parameter to PostReview (was
  removed during PR #112 refactoring)
- gitea/adapter.go: Forward req.CommitID to underlying client
- github/review.go: Use req.CommitID as primary anchor, fall back to
  comment-derived CommitID when empty, reject on conflict
- cmd/review-bot/main.go: Set ReviewRequest.CommitID = evaluatedSHA

Fixes #115

@@ -217,3 +250,4 @@
+}
 func TestAdapter_PostReview_WithComments_PositionTranslation(t *testing.T) {
 	diff := `diff --git a/main.go b/main.go
 --- a/main.go

@@ -94,5 +94,10 @@ type ReviewRequest struct {
 	Body string `json:"body"`
 	// Event is the review action (approve, request changes, or comment).
 	Event    ReviewEvent     `json:"event"`
+	// CommitID anchors the review to a specific commit SHA.

@@ -217,3 +249,4 @@
+}
 func TestAdapter_PostReview_WithComments_PositionTranslation(t *testing.T) {
 	diff := `diff --git a/main.go b/main.go
 --- a/main.go

@@ -94,5 +94,10 @@ type ReviewRequest struct {
 	Body string `json:"body"`
 	// Event is the review action (approve, request changes, or comment).
 	Event    ReviewEvent     `json:"event"`
+	// CommitID anchors the review to a specific commit SHA.

@@ -100,3 +102,4 @@
+	// specifies a different non-empty CommitID, we reject the request.
 	for _, comment := range req.Comments {
 		if comment.CommitID != "" {
 			if payload.CommitID == "" {

@@ -94,5 +94,10 @@ type ReviewRequest struct {
 	Body string `json:"body"`
 	// Event is the review action (approve, request changes, or comment).
 	Event    ReviewEvent     `json:"event"`
+	// CommitID anchors the review to a specific commit SHA.

@@ -182,3 +182,3 @@
 	}
 
-	review, err := a.client.PostReview(ctx, owner, repo, number, event, req.Body, giteaComments)
+	review, err := a.client.PostReview(ctx, owner, repo, number, event, req.Body, req.CommitID, giteaComments)

@@ -97,2 +100,4 @@
+	// head; GitHub derives from comments or omits).
+	CommitID string          `json:"commit_id,omitempty"`
 	Comments []ReviewComment `json:"comments,omitempty"`
 }

@@ -97,0 +96,4 @@
+	Event ReviewEvent `json:"event"`
+	// CommitID anchors the review to a specific commit SHA.
+	// Both GitHub and Gitea accept this at the review level.
+	// If empty, each platform applies its default behavior (Gitea uses PR

@@ -214,6 +214,40 @@ func TestAdapter_PostReview_EventTranslation(t *testing.T) {
 	}

@@ -99,4 +99,3 @@
 	// Adapters use this as the primary commit anchor for the review submission.
 	CommitID string `json:"commit_id,omitempty"`
-
 	Comments []ReviewComment `json:"comments,omitempty"`

@@ -486,3 +486,3 @@
 		Body:     reviewBody,
 		Event:    event,
-		CommitID: pr.Head.SHA,
+		CommitID: evaluatedSHA,

@@ -147,6 +147,46 @@ func TestPostReview(t *testing.T) {
 	}