connection-state: write the host state record atomically (temp + rename) #808

Follow-up from the #794 review (non-blocking, pre-existing):

writeHostState (packages/cli/src/utils/connection-state.ts) writes the host state record in place. Write it to a temp file in the same directory and rename it over the record, so a failed write leaves the previous record intact rather than a partial one. Keep the 0600 mode on the temp file before the rename. Test: a write that fails after the temp is created leaves the old record byte-identical and no temp file behind.

Refs #794.