Real E2E update/uninstall + CBM_DOWNLOAD_URL support

Add CBM_DOWNLOAD_URL env var to cbm_cmd_update and checksum
verification — allows E2E testing against local HTTP server.

Phase 14: runs actual update --standard -y against local server,
verifies binary replaced, agent configs refreshed (stale path
updated), then real uninstall -y verifies binary removed and
configs cleaned.

Phase 13: add PATH setup verification. Non-interactive stdin:
isatty() prevents silent hangs.