feat(delivery): final report + reserved-label guard + Windows test fixes (v2 phase A3-final)

Gap-closing on top of the merged A3-3..A3-6 work:

- ProjectService.create_preview_domain now refuses a label that is a reserved
  platform name (git, mcp, api, panel, ...) via PROTECTED_HOST_LABELS. The rules
  set existed but only the owner-facing validate_extra_host consulted it, so the
  tenant path could reserve one.
- process.py: signal.SIGKILL is resolved through getattr with a SIGTERM
  fallback, so the module imports and type-checks on Windows where SIGKILL does
  not exist.
- Three more Windows test repairs: a git-created .git must be unlinked and
  recreated rather than overwritten in place; POSIX-only chmod and symlink
  assertions are gated on os.name (the security property they assert is checked
  everywhere, the platform-specific mechanics only where they apply).
- docs/plan/reports/agent-3-final.md: the integration hand-off — host
  requirements, every setting, every endpoint added or changed, the tables, the
  open CHANGE REQUESTS, a 12-step manual QA script and the known limitations.

Checks: ruff clean, mypy clean (136 files), 521 API tests pass, 83 web tests
pass, tsc clean, next lint clean.

apps/api/app/application/services/project_service.py

@@ -32,6 +32,7 @@ from app.core.crypto import decrypt, encrypt
 from app.core.errors import ConflictError, NotFoundError, PermissionDeniedError, ValidationError
 from app.domain.base import ensure_unique_slug, slugify
 from app.domain.platform.rules import (
+    PROTECTED_HOST_LABELS,
     assert_not_protected_port,
     assert_port_allocatable,
     build_host,
@@ -195,6 +196,13 @@ class ProjectService:
         port: int | None = None,
     ) -> dict:
         label = validate_subdomain_label(requested_subdomain)
+        # ``git``/``mcp``/``api``/``panel``… are platform services. A tenant host
+        # must never be able to shadow one on the same root domain — the rules
+        # module defines the reserved set (see PROTECTED_HOST_LABELS).
+        if label in PROTECTED_HOST_LABELS:
+            raise PermissionDeniedError(
+                f"نام «{label}» برای سرویس‌های سامانه رزرو شده است."
+            )
         host = build_host(label, settings.preview_root_domain)
 
         if await self.domains.get_by_host(host) is not None: