feat(migration): phase 5b — STRIX_USE_SDK_HARNESS dispatch flag

Adds the env-var gate that lets users opt into the SDK harness without
disturbing the legacy default. Per PLAYBOOK §7.1, this is the cutover
mechanism: STRIX_USE_SDK_HARNESS=1 routes scans through run_strix_scan
(the Phase 5 entry point); anything else continues to use
StrixAgent.execute_scan.

- strix/interface/sdk_dispatch.py:
  - should_use_sdk_harness(): truthy-string parse of the env var.
  - _resolve_sandbox_image(): reads strix_image from Config; falls
    back to "strix-sandbox:latest" with a warning if unset.
  - _resolve_sources_path(): when --local-sources is given, mounts
    its parent so the agent walks down to the source tree; otherwise
    creates a per-run scratch dir under XDG_CACHE_HOME/strix/sources/.
    Phase 6 will replace this with the legacy clone-into-container
    flow once we port that.
  - run_scan_via_sdk(): the adapter — translates the legacy CLI
    (scan_config dict + argparse Namespace + Tracer) into the keyword
    arguments run_strix_scan expects. Returns the SDK RunResult; lets
    failures bubble up.

- strix/interface/cli.py: adds the dispatch branch inside the existing
  Live/status loop. Legacy default unchanged; SDK path is reached only
  when STRIX_USE_SDK_HARNESS is truthy. Two pre-existing lazy imports
  hoisted to module level (cleanup_runtime + sdk_dispatch helpers) so
  ruff is happy.

Pre-existing legacy lint/type issues surfaced when pre-commit checked
the edited cli.py and chased imports — fixed or ignored in passing:
- utils.py:1052 duplicate ``metadata`` annotation removed.
- utils.py:1251 unused ``# type: ignore[import-not-found]`` for yarl.
- main.py:456 ``panel_parts`` inferred type rejected later string
  entries — explicit ``list[Text | str]`` annotation.
- utils.py:resolve_diff_scope_context PLR0912 (16 branches) per-file
  ignore — branches map 1:1 to scope-mode × target-type combinations.

Tests: 18 new tests in tests/interface/test_sdk_dispatch.py — env
flag parsing parametrized over truthy/falsy variants, image lookup
with config hit + miss-with-warning, sources path resolution for
local_sources / alternative key names / scratch-dir creation, and
the adapter's kwarg handoff verified against a patched
run_strix_scan (run_name from args + run_name from scan_config
fallback + failure propagation).

Refs: PLAYBOOK.md §7.1 (cutover), §7.2 (rollback).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -312,6 +312,10 @@ ignore = [
 # walks every supported target type — splitting it into per-type
 # helpers would add indirection without simplifying anything.
 "strix/sdk_entry.py" = ["TC003", "PLR0912"]
+# Legacy interface utility with intentionally many branches per supported
+# scope-mode / target-type combination; refactor would obscure the
+# decision tree without simplifying it.
+"strix/interface/utils.py" = ["PLR0912"]
 # Sandbox dispatch helper has many short-circuit error returns (auth fail,
 # size cap, decode fail, etc). Each is a distinct, documented failure mode
 # the model needs to see verbatim — collapsing them harms readability.