fix(web): require clone dir strictly under build_vms on remove (W6)

remove_vm() deletes vmx.parent, so containment must check the clone dir, not just the vmx: a crafted vmx_id of build_vms/x.vmx passed the old check and would rmtree the entire clone base. Now the parent must be strictly inside build_vms.

plans/web-ui-management-plan.md

@@ -24,7 +24,7 @@ Stato passi (spunta man mano):
 - [x] **W3** — `operations/audit.py`: append-only JSONL + lettura. Config `[web]` in `config.py`.
 - [x] **W4** — App web core: `web/app.py` factory + `commands/web.py` (`ci web serve`). Dashboard **read-only** (VM, cache, disco, runner). Ancora **senza auth**, bind localhost.
 - [x] **W5** — Auth: Gitea OIDC (Authlib) + sessione firmata + allowlist utenti (fail-closed) + secret solo da keyring. Tutte le route dati protette (`/healthz`,`/static`,`/login`,`/auth/callback` aperte). Hardening: `ci web serve` rifiuta bind non-loopback quando `[web.oidc]` assente (open mode solo su localhost).
-- [ ] **W6** — Azioni mutanti + safety: remove VM, cleanup orfane, purge cache, backup. Confirm gate + CSRF + audit + dry-run. HTMX partials.
+- [x] **W6** — Azioni mutanti + safety: remove VM, cleanup orfane, purge cache, backup. Confirm gate 2-step (preview dry-run → execute) + CSRF sincronizzatore + audit (preview+execute) + path-containment (clone dir **strettamente** sotto build_vms) + 409 job-race guard. `SessionMiddleware` sempre attivo (secret effimero in open mode) per CSRF. HTMX partials.
 - [ ] **W7** — UI dark Gitea-style (asset vendorizzati) + installer ripetibile (`setup-web-linux.sh`, unit systemd, `docs/WEB-UI.md`) + validazione finale (test, coverage, mypy, ruff, smoke manuale).
 
 **Definition of done globale**: `ci web serve` gira come `ci-web.service`, login via Gitea, dashboard dark mostra stato live, le 4 azioni (kill VM / cleanup orfane / purge cache / backup) funzionano con conferma + audit, suite test verde con coverage ≥ 90 % e `mypy --strict` pulito.