feat(harnessx): configurable provider + per-task budget caps

Add a [harnessx] config block so the agentic plane can be steered from
openultrasast.toml, and make the LLM provider selectable (the hunter and verifier
previously hardcoded AnthropicProvider).

- config.HarnessxConfig: provider ("anthropic"|"openai"|"litellm", default
  anthropic), max_cost_usd (2.0), token_threshold (120000); parsed from [harnessx].
- harness_ext.build_provider: single lazy, capability-guarded provider factory.
  The provider name is validated before any import, so an unknown choice fails with
  a clear ValueError even when the extra is absent; each provider reads its own
  standard key env var (openai -> OPENAI_API_KEY, anthropic -> ANTHROPIC_API_KEY).
- HxScanOrchestrator and verify_findings_dispatch take a provider and route through
  build_provider; cli threads config.harnessx.{provider,max_cost_usd,token_threshold}
  into both the hunter and the verifier.
- README: "Configuring the HarnessX agentic plane" section (install, models,
  provider, budgets, fallback/degradation).

Test isolation: convert the "harnessx not in sys.modules" import-guards to a
hermetic subprocess fixture (tests/conftest.py assert_cold_of_harnessx) and
simulate extra-absent in the CLI dispatch tests, so the suite is deterministic
whether or not the optional extra is installed (it now is, locally). Verified the
OpenAI path constructs end-to-end (build_provider -> OpenAIProvider; orchestrator
-> Harness) with the budget threaded through.

137 passed, 3 skipped; ruff + mypy clean; detection gate PASS.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

tests/test_harness_ext.py

@@ -1,13 +1,19 @@
 """HarnessX optional-extra packaging + capability guard (Phase 3 task 5.2)."""
 
 import re
-import sys
 import tomllib
 from pathlib import Path
 
 import pytest
 
-from openultrasast.harness_ext import HARNESSX_EXTRA, HarnessXUnavailableError, has_harnessx, require_harnessx
+from openultrasast.harness_ext import (
+    HARNESSX_EXTRA,
+    SUPPORTED_PROVIDERS,
+    HarnessXUnavailableError,
+    build_provider,
+    has_harnessx,
+    require_harnessx,
+)
 
 PYPROJECT = tomllib.loads(Path("pyproject.toml").read_text())
 
@@ -25,18 +31,15 @@ def test_harnessx_is_a_sha_pinned_optional_extra() -> None:
     ), extras[0]
 
 
-def test_importing_openultrasast_does_not_import_harnessx() -> None:
+def test_importing_openultrasast_does_not_import_harnessx(assert_cold_of_harnessx) -> None:  # type: ignore[no-untyped-def]
     # The big modules (cli, harness_ext) must not eagerly import the heavy extra.
-    import openultrasast.cli  # noqa: F401
-    import openultrasast.harness_ext  # noqa: F401
-
-    assert "harnessx" not in sys.modules
+    assert_cold_of_harnessx("import openultrasast.cli\nimport openultrasast.harness_ext")
 
 
-def test_has_harnessx_probes_without_importing() -> None:
-    available = has_harnessx()
-    assert isinstance(available, bool)
-    assert "harnessx" not in sys.modules  # find_spec must not import the module
+def test_has_harnessx_probes_without_importing(assert_cold_of_harnessx) -> None:  # type: ignore[no-untyped-def]
+    assert isinstance(has_harnessx(), bool)
+    # find_spec must not import the module.
+    assert_cold_of_harnessx("from openultrasast.harness_ext import has_harnessx\nassert isinstance(has_harnessx(), bool)")
 
 
 def test_require_harnessx_raises_with_install_guidance_when_absent() -> None:
@@ -44,3 +47,27 @@ def test_require_harnessx_raises_with_install_guidance_when_absent() -> None:
         pytest.skip("HarnessX extra is installed in this environment")
     with pytest.raises(HarnessXUnavailableError, match=re.escape(HARNESSX_EXTRA)):
         require_harnessx()
+
+
+def test_build_provider_rejects_unknown_provider_before_importing(assert_cold_of_harnessx) -> None:  # type: ignore[no-untyped-def]
+    # Name is validated before any import, so this fails loudly even without the extra.
+    with pytest.raises(ValueError, match="unknown HarnessX provider"):
+        build_provider("gpt-4o", "bogus")
+    assert_cold_of_harnessx(
+        "from openultrasast.harness_ext import build_provider\n"
+        "try:\n    build_provider('gpt-4o', 'bogus')\n"
+        "except ValueError:\n    pass\n"
+        "else:\n    raise SystemExit('expected ValueError')\n"
+    )
+
+
+def test_supported_providers_cover_anthropic_and_openai() -> None:
+    assert {"anthropic", "openai"} <= set(SUPPORTED_PROVIDERS)
+
+
+@pytest.mark.parametrize(("provider", "class_name"), [("anthropic", "AnthropicProvider"), ("openai", "OpenAIProvider")])
+def test_build_provider_constructs_selected_provider(provider: str, class_name: str) -> None:
+    if not has_harnessx():
+        pytest.skip("HarnessX extra not installed in this environment")
+    built = build_provider("test-model", provider)
+    assert type(built).__name__ == class_name