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/slim.go

@@ -184,6 +184,28 @@ func slimContents(c *gitea_sdk.ContentsResponse) map[string]any {
 	return m
 }
 
+func slimTree(t *gitea_sdk.GitTreeResponse) map[string]any {
+	if t == nil {
+		return nil
+	}
+	entries := make([]map[string]any, 0, len(t.Entries))
+	for _, e := range t.Entries {
+		entries = append(entries, map[string]any{
+			"path": e.Path,
+			"mode": e.Mode,
+			"type": e.Type,
+			"size": e.Size,
+			"sha":  e.SHA,
+		})
+	}
+	return map[string]any{
+		"sha":         t.SHA,
+		"truncated":   t.Truncated,
+		"total_count": t.TotalCount,
+		"tree":        entries,
+	}
+}
+
 func slimDirEntries(entries []*gitea_sdk.ContentsResponse) []map[string]any {
 	out := make([]map[string]any, 0, len(entries))
 	for _, c := range entries {