Per-fixture tool allowlist unblocks measuring nudges that need Edit/Write
(e.g. literal-interpretation 'fix the failing tests' needs write access).
Per-fixture maxTurns lets harder prompts run longer without changing the
default. `direction` is cosmetic metadata for test output labeling.
Also adds reusable predicates and metrics:
- lowerIsBetter20Pct / higherIsBetter20Pct — 20% lift threshold vs baseline
- bashToolCallCount — count of Bash tool_use across the session
- turnsToCompletion — SDK-reported num_turns at result
- uniqueFilesEdited — Edit/Write/MultiEdit file_path set size
test/skill-e2e-overlay-harness.test.ts now threads fixture.allowedTools
and fixture.maxTurns through runArm.
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