v0.3.0 — session lifecycle (D3)

Sessions are now first-class, discoverable, multiplexable, and garbage-collected.

• tjor ls — every session on the machine, with a live internal-only re-check that flags a tampered session as DEGRADED
• tjor attach [session] — reattach to a running agent (picker when ambiguous; detach with ctrl-p ctrl-q)
• tjor run --session <name> — concurrent, fully isolated sessions in one repo (own topology, state, identity); --detach starts without attaching
• Agents now launch detached and persistent — a dropped terminal leaves a reattachable session instead of deleting it
• tjor gc [--age] [--dry-run] — reap idle containers/networks/volumes (label-selected only, never session state)
• tjor reset {cache|sessions|creds|all} [--dry-run] — tiered, symlink-safe state wipe
• Also fixed: root-owned XDG state on a native-Linux engine (state is now host-manageable everywhere)

New CI job runs the 28-check lifecycle integration test live on the Linux engine. 103 unit/parity tests, 15/15 conformance.