Replaces the in-memory database with an on-disk project. Startup either
opens a project at the positional CLI path (L1) or creates an auto-named
temp project (<YYYYMMDD>-<word>-<word>-<word>) under the OS-standard
data directory or a --data-dir override. The new project::Project type
owns the directory skeleton and a PID+hostname lock file with
stale-lock takeover via sysinfo. The status bar now shows
"Project: <Display Name>", derived by a small kebab/snake/camel
prettifier. Per-command persistence to YAML/CSV/history.log is NOT
yet wired -- that's Iteration 2; for now playground.db carries the
state across quits.
Tests: 257 passing (231 lib + 9 new integration + 17 existing),
0 failing, 0 skipped. Clippy clean with nursery lints.
claude@clouddev1