feat(eval): extend OverlayFixture with allowedTools, maxTurns, direction

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.

test/skill-e2e-overlay-harness.test.ts

@@ -141,8 +141,8 @@ async function runArm(
           userPrompt: fixture.userPrompt,
           workingDirectory: dir,
           model: fixture.model,
-          maxTurns: 5,
-          allowedTools: ['Read', 'Glob', 'Grep', 'Bash'],
+          maxTurns: fixture.maxTurns ?? 5,
+          allowedTools: fixture.allowedTools ?? ['Read', 'Glob', 'Grep', 'Bash'],
           permissionMode: 'bypassPermissions',
           settingSources: [],
           env: { ANTHROPIC_API_KEY: process.env.ANTHROPIC_API_KEY ?? '' },