test(typecheck): tests/**/* uncovered by tsc

[dean0x]

Problem

tsconfig.json:19 sets "include": ["src/**/*"] and vitest.config.ts has no typecheck block. Consequence: nothing in CI typechecks tests/.

This gap surfaced in PR #316 (feat/315-gate-attribution-flag): the headline compile-time guarantee of the EnvBooleanFlagDef | SettingBooleanFlagDef split cannot be locked in with a regression test (// @ts-expect-error), and a stray third argument in a makeTemplate(...) call went unnoticed until resolve pass. Without test typechecking, these slip through.

Current Status

• Pre-existing repo config gap: identified in PR feat: gate AI-attribution suppression behind typed flag with init wizard #316 review (2026-09-01)
• Scope outside this PR: enabling test typechecking surfaces unrelated errors across the whole tests/ tree, out of scope for feat: gate AI-attribution suppression behind typed flag with init wizard #316
• Confidence: 90% (from typescript review)
• Related PF: avoids PF-018 (test coverage contract)

Expected Behavior

• Typechecker covers both src/**/* and tests/**/*
• Compile-time guarantees enforced in test assertions
• Runtime backstops (as Type casts) replaced with // @ts-expect-error regression tests
• All type errors surfaced by CI before merge

Suggested Approach

Add a tsconfig.test.json (or vitest typecheck block) covering tests/**, run it in CI, and burn down the errors it surfaces.

Acceptance Criteria

• TypeScript config covers tests/**/*
• Typecheck runs in CI
• Test suite passes typecheck without errors
• Regression test gaps (// @ts-expect-error) documented
• Runtime casts replaced with compile-time assertions where possible

Related

• PR feat: gate AI-attribution suppression behind typed flag with init wizard #316 (feat/315-gate-attribution-flag)
• Source: typescript.md review finding, 2026-09-01
• Ref: tsconfig.json:19, vitest.config.ts
• Related files: tests/post-install-merge.test.ts (unrelated errors uncovered)