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/search/slim_test.go

@@ -0,0 +1,54 @@
+package search
+
+import (
+	"slices"
+	"testing"
+
+	gitea_sdk "code.gitea.io/sdk/gitea"
+)
+
+func TestSlimIssues(t *testing.T) {
+	issues := []*gitea_sdk.Issue{
+		{
+			Index:       1,
+			Title:       "Bug report",
+			State:       gitea_sdk.StateOpen,
+			HTMLURL:     "https://gitea.com/org/repo/issues/1",
+			Poster:      &gitea_sdk.User{UserName: "alice"},
+			Labels:      []*gitea_sdk.Label{{Name: "bug"}},
+			Repository:  &gitea_sdk.RepositoryMeta{FullName: "org/repo"},
+			PullRequest: nil,
+		},
+		{
+			Index:       2,
+			Title:       "Add feature",
+			State:       gitea_sdk.StateOpen,
+			Poster:      &gitea_sdk.User{UserName: "bob"},
+			Repository:  &gitea_sdk.RepositoryMeta{FullName: "org/repo"},
+			PullRequest: &gitea_sdk.PullRequestMeta{},
+		},
+	}
+
+	result := slimIssues(issues)
+	if len(result) != 2 {
+		t.Fatalf("expected 2 issues, got %d", len(result))
+	}
+	if result[0]["repository"] != "org/repo" {
+		t.Errorf("expected repository org/repo, got %v", result[0]["repository"])
+	}
+	if result[0]["labels"].([]string)[0] != "bug" {
+		t.Errorf("expected label bug, got %v", result[0]["labels"])
+	}
+	if _, ok := result[0]["is_pull"]; ok {
+		t.Error("issue should not have is_pull")
+	}
+	if result[1]["is_pull"] != true {
+		t.Error("PR should have is_pull=true")
+	}
+}
+
+func TestSearchIssuesToolRequired(t *testing.T) {
+	if !slices.Contains(SearchIssuesTool.InputSchema.Required, "query") {
+		t.Error("search_issues should require query")
+	}
+}