Add get_commit, get_repository_tree, and search_issues tools (#162)

Add three new read-only tools inspired by the GitHub MCP server:

- `get_commit`: Get details of a specific commit by SHA, branch, or tag
- `get_repository_tree`: Get the file tree of a repository with optional recursive traversal, pagination, and ref support
- `search_issues`: Search issues and pull requests across all accessible repositories with filters for state, type, labels, and owner

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/162
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

operation/repo/tree_test.go

@@ -0,0 +1,52 @@
+package repo
+
+import (
+	"slices"
+	"testing"
+
+	gitea_sdk "code.gitea.io/sdk/gitea"
+)
+
+func TestSlimTree(t *testing.T) {
+	tree := &gitea_sdk.GitTreeResponse{
+		SHA:        "abc123",
+		TotalCount: 2,
+		Truncated:  false,
+		Entries: []gitea_sdk.GitEntry{
+			{Path: "src", Mode: "040000", Type: "tree", Size: 0, SHA: "def456"},
+			{Path: "main.go", Mode: "100644", Type: "blob", Size: 42, SHA: "789abc"},
+		},
+	}
+
+	m := slimTree(tree)
+	if m["sha"] != "abc123" {
+		t.Errorf("expected sha abc123, got %v", m["sha"])
+	}
+	if m["total_count"] != 2 {
+		t.Errorf("expected total_count 2, got %v", m["total_count"])
+	}
+	entries := m["tree"].([]map[string]any)
+	if len(entries) != 2 {
+		t.Fatalf("expected 2 entries, got %d", len(entries))
+	}
+	if entries[0]["path"] != "src" {
+		t.Errorf("expected first entry path src, got %v", entries[0]["path"])
+	}
+	if entries[1]["type"] != "blob" {
+		t.Errorf("expected second entry type blob, got %v", entries[1]["type"])
+	}
+}
+
+func TestSlimTreeNil(t *testing.T) {
+	if m := slimTree(nil); m != nil {
+		t.Errorf("expected nil, got %v", m)
+	}
+}
+
+func TestGetRepoTreeToolRequired(t *testing.T) {
+	for _, field := range []string{"owner", "repo", "tree_sha"} {
+		if !slices.Contains(GetRepoTreeTool.InputSchema.Required, field) {
+			t.Errorf("expected %q to be required", field)
+		}
+	}
+}