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_hunter_harness.py

@@ -1,17 +1,13 @@
 """HarnessX hunter sub-harness: guarding, findings parsing, construction smoke (task 6.1)."""
 
-import sys
-
 import pytest
 
 from openultrasast.harness_ext import HarnessXUnavailableError, build_sast, has_harnessx
 from openultrasast.hunter_harness import HxScanOrchestrator, _extract_findings
 
 
-def test_module_import_does_not_pull_harnessx() -> None:
-    import openultrasast.hunter_harness  # noqa: F401
-
-    assert "harnessx" not in sys.modules
+def test_module_import_does_not_pull_harnessx(assert_cold_of_harnessx) -> None:  # type: ignore[no-untyped-def]
+    assert_cold_of_harnessx("import openultrasast.hunter_harness")
 
 
 def test_extract_findings_parses_json_array_and_tolerates_garbage() -> None: