Hierarchical KiCad projects store sub-sheets in subdirectories (e.g.
sheets/). The flat os.listdir scan missed all of them. Use recursive
glob to find .kicad_sch files at any depth under the project directory.
Reported by ESP32-P4 project (agent thread message 025) — their 8
malformed property-private entries were all in sheets/ subdirectory.
validate_project now scans all .kicad_sch files for two classes of
silent rendering failure: quoted (property "private" ...) that should
be a bare keyword in KiCad 9, and lib_id references with no matching
lib_symbols entry. Both cause kicad-cli to blank entire pages during
export without any error message.
Also pass working_dir to run_kicad_command in export_pdf for robust
library path resolution.
Addresses feedback from ESP32-P4 project (agent thread message 021).
