Implements the Gitea adapter (gitea.Adapter) that satisfies vcs.Client.

Key components:
- gitea/adapter.go: Adapter struct wrapping *Client with all vcs.Client methods
- gitea/position.go: BuildPositionToLineMap for diff-position → line translation
- gitea/adapter_test.go: Tests for all mapping methods and event translation
- gitea/position_test.go: Tests for position translation edge cases

Translation details:
- ReviewEvent: APPROVE → APPROVED (Gitea-native)
- PostReview: fetches diff, builds position map, translates each comment
- Deletion-targeted positions map to nearest non-deletion line below
- All field-mapping methods tested (GetPullRequest, GetPullRequestFiles,
  ListReviews, GetCommitStatuses, ListContents)

Also:
- Added Base field to gitea.PullRequest struct
- Updated conformance tests to assert Adapter (not raw Client) satisfies vcs.Client
- Removed phase2 build tag from conformance tests

Closes #79

@@ -0,0 +150,4 @@
+		// Fetch diff to build position → line number map
+		diff, err := a.client.GetPullRequestDiff(ctx, owner, repo, number)
+		if err != nil {
+			return nil, fmt.Errorf("fetch diff for position translation: %w", err)

@@ -0,0 +179,4 @@
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var gotEvent string

@@ -0,0 +341,4 @@
+	}
+	if len(entries) != 2 {
+		t.Fatalf("got %d entries, want 2", len(entries))
+	}

@@ -0,0 +57,4 @@
+	max := 0
+	for pos := range pm.files[file] {
+		if pos > max {
+			max = pos

@@ -0,0 +77,4 @@
+
+	lines := strings.Split(diff, "\n")
+	var currentFile string
+	var position int

@@ -0,0 +54,4 @@
+func (a *Adapter) GetPullRequestDiff(ctx context.Context, owner, repo string, number int) (string, error) {
+	return a.client.GetPullRequestDiff(ctx, owner, repo, number)
+}
+

@@ -0,0 +126,4 @@
+// --- Reviewer ---
+
+// translateEvent translates a vcs.ReviewEvent (GitHub-canonical) to a Gitea-native event string.
+func translateEvent(event vcs.ReviewEvent) string {

@@ -0,0 +57,4 @@
+	max := 0
+	for pos := range pm.files[file] {
+		if pos > max {
+			max = pos

@@ -0,0 +29,4 @@
+}
+
+// --- PRReader ---
+

@@ -0,0 +149,4 @@
+	event := translateEvent(req.Event)
+
+	var giteaComments []ReviewComment
+	if len(req.Comments) > 0 {

@@ -0,0 +277,4 @@
+		},
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)

@@ -0,0 +51,4 @@
+
+	return lineNum, nil
+}
+

@@ -0,0 +104,4 @@
+			continue
+		}
+
+		// Skip diff --git lines

@@ -0,0 +137,4 @@
+	default:
+		// Unknown events pass through as-is. This is intentional: new event types
+		// added to vcs.ReviewEvent will still be forwarded without a code change here,
+		// and Gitea will reject truly invalid values with a clear API error.

@@ -0,0 +167,4 @@
+	}
+}
+
+func TestAdapter_PostReview_EventTranslation(t *testing.T) {

@@ -0,0 +46,4 @@
+				return ln, nil
+			}
+		}
+		return 0, fmt.Errorf("position %d targets a deletion line with no subsequent new-file line in %q", position, file)

@@ -0,0 +37,4 @@
+		return nil, err
+	}
+	return &vcs.PullRequest{
+		Number: number,

@@ -0,0 +190,4 @@
+		State:    review.State,
+		Stale:    review.Stale,
+		CommitID: review.CommitID,
+	}, nil

@@ -0,0 +216,4 @@
+
+func TestAdapter_PostReview_WithComments_PositionTranslation(t *testing.T) {
+	diff := `diff --git a/main.go b/main.go
+--- a/main.go

@@ -0,0 +87,4 @@
+		// non-overlapping ("+++ /dev/null" does not start with "+++ b/"), so
+		// ordering is independent. Checking the common case first for clarity.
+		if strings.HasPrefix(line, "+++ b/") {
+			currentFile = strings.TrimPrefix(line, "+++ b/")

@@ -0,0 +141,4 @@
+		return string(event)
+	}
+}
+

@@ -0,0 +123,4 @@
+	return result, nil
+}
+
+// --- Reviewer ---

@@ -0,0 +181,4 @@
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var gotEvent string
+			server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {

@@ -0,0 +72,4 @@
+//   - Deletion lines have a position but no new-file line number (stored as -1)
+func BuildPositionToLineMap(diff string) *PositionMap {
+	pm := &PositionMap{
+		files:        make(map[string]map[int]int),

@@ -0,0 +97,4 @@
+		}
+
+		// Deleted file: +++ /dev/null means the file is being deleted
+		if strings.HasPrefix(line, "+++ /dev/null") {

@@ -0,0 +76,4 @@
+		maxPositions: make(map[string]int),
+	}
+
+	lines := strings.Split(diff, "\n")