Build 97

fix: fall back to runner cwd when provided cwd invalid (os error 267)

spawn_process and spawn_exec_process previously passed cwd directly
to Command::current_dir(), which on Windows fails with ERROR_DIRECTORY
(os error 267) if the path does not exist. This surfaced as cascading
runner errors where every exec: tool returned 'The directory name is
invalid', leaving the runner unusable until restart.

Adds resolve_valid_cwd() — validates path is a directory, falls back
to runner's own current_dir with a warning log. Covers both the child
exec-process spawn (rpc.rs) and every runtime branch (runtime.rs).