feat(harness): derive a 3-5 keyword session name (YAKE, no LLM)

New sessions get a concise Italian-keyword `name` instead of the truncated
question. `tht session new` (when no --name is given) derives it via a new
`_extract_name` helper using YAKE (pure-Python, unsupervised, Italian, no LLM),
dropping generic query verbs and keeping the top keywords in reading order;
falls back to `_summarize` if YAKE is unavailable. `create_session` core keeps
its `name=None` default — the policy lives at the CLI layer.

TDD: tests/test_session_name.py (unit + CliRunner integration). Full harness
suite 269 passed; ruff clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

harness/tht/cli/session_cmd.py

@@ -78,11 +78,12 @@ def new_cmd(
     config: Path = CONFIG_OPT,
 ) -> None:
     """Crea una sessione e stampa il suo id (ultima riga dell'output)."""
-    from tht.session.store import create_session
+    from tht.session.store import _extract_name, create_session
 
     cfg = _load_config_or_exit(config)
     manifest = create_session(question, cfg.database, cfg.paths.sessions,
-                              provider=provider, model=model, thinking=thinking, name=name)
+                              provider=provider, model=model, thinking=thinking,
+                              name=name or _extract_name(question))
     if json_out:
         typer.echo(json.dumps({"id": manifest.id}, ensure_ascii=False))
         return