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/conftest.py

@@ -0,0 +1,33 @@
+"""Shared test fixtures."""
+
+import subprocess
+import sys
+
+import pytest
+
+_HARNESSX_GUARD = (
+    "\nimport sys as _sys\n"
+    "_leaked = sorted(m for m in _sys.modules if m == 'harnessx' or m.startswith('harnessx.'))\n"
+    "assert not _leaked, 'leaked harnessx modules: ' + repr(_leaked)\n"
+)
+
+
+@pytest.fixture
+def assert_cold_of_harnessx():  # type: ignore[no-untyped-def]
+    """Run ``code`` in a fresh interpreter and assert it imported no ``harnessx`` module.
+
+    Hermetic by construction: the lazy-import guarantee is verified the same way
+    whether or not the optional extra is installed in this environment (a global
+    ``sys.modules`` check would be polluted by other tests once the extra is present).
+    """
+
+    def _check(code: str) -> str:
+        proc = subprocess.run(
+            [sys.executable, "-c", code + _HARNESSX_GUARD],
+            capture_output=True,
+            text=True,
+        )
+        assert proc.returncode == 0, f"exit={proc.returncode}\nstdout={proc.stdout}\nstderr={proc.stderr}"
+        return proc.stdout
+
+    return _check