deterministic Claude session naming, resume, slots, and listing
vrg-vm session now launches Claude Code by default with a deterministic name ::, auto-resuming the lowest idle slot or creating the next free one. Adds --slot/--fork and an in-VM resolver (vrg-vm-resolve-session) that classifies sessions from the VM-local roster plus shared transcripts and execs claude with -n/--resume/--fork-session. Adds vrg-vm list --sessions across all identity VMs. Pure logic in lib/session.py; raw '-- ' override preserved. Implements #1292; design in vergil-vm#73. Ref #1292.