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_config.py

@@ -28,6 +28,33 @@ def test_load_config_reads_toml(tmp_path: Path) -> None:
     assert config.dynamic.network_scope == ("127.0.0.1:8080",)
 
 
+def test_harnessx_defaults_when_section_absent() -> None:
+    config = load_config(None)
+    assert config.harnessx.provider == "anthropic"
+    assert config.harnessx.max_cost_usd == 2.0
+    assert config.harnessx.token_threshold == 120_000
+
+
+def test_load_config_reads_harnessx_section(tmp_path: Path) -> None:
+    config_path = tmp_path / "openultrasast.toml"
+    config_path.write_text(
+        "\n".join(
+            [
+                "[harnessx]",
+                'provider = "openai"',
+                "max_cost_usd = 5.0",
+                "token_threshold = 200000",
+            ]
+        )
+    )
+
+    config = load_config(config_path)
+
+    assert config.harnessx.provider == "openai"
+    assert config.harnessx.max_cost_usd == 5.0
+    assert config.harnessx.token_threshold == 200_000
+
+
 def test_write_resolved_config_creates_json(tmp_path: Path) -> None:
     output = tmp_path / "run" / "resolved_config.json"