test: integration tests for validate_and_load and state lifecycle

[snipcodeit]

Summary

• Adds test/validate-and-load.test.js with 23 vitest integration tests across 5 groups covering the full validate_and_load pattern in lib/state.cjs
• All tests isolated in tmp directories via process.cwd() override — no real .mgw/ pollution
• migrateProjectState() idempotency verified with double-run assertions
• Staleness detection tested via mock-github.cjs intercepting child_process.execSync
• Includes vitest infrastructure (config, setup, fixture loader, 16 fixture files) from PRs Create lib/mock-github.cjs with GitHub API interceptors for tests #247 and feat: add vitest test runner with fixture loader and mock activation #260

Closes #250

Milestone Context

• Milestone: v6 — Local Testing & Simulation Framework
• Phase: Phase 48: State & Pipeline Tests (issue 4/4 in milestone)
• Dependencies: Blocked by: Add vitest test runner with fixture loader and mock activation #249 (vitest) → PR feat: add vitest test runner with fixture loader and mock activation #260

Changes

New Files

• test/validate-and-load.test.js — 23 vitest integration tests (5 groups)
• test/fixtures/github/ — 16 fixture JSON files for mock-github.cjs activation
• test/setup.js — Vitest global setup with mock auto-activation
• test/loadFixture.js — Fixture loader helper
• vitest.config.js — Vitest config targeting .test.js / .spec.js
• lib/mock-github.cjs — GitHub API interceptor for tests

Modified Files

• package.json — Added vitest devDependency and test scripts (test, test:watch, test:node)

Test Groups

Group	Tests	Coverage
1: Fresh .mgw/ init	5	getMgwDir, writeProjectState, storeDependencies, gitignore idempotency
2: migrateProjectState idempotency	5	field injection, double-run safety, milestone sub-fields, active file fields
3: loadActiveIssue lifecycle	4	null returns, prefix pattern, string coercion
4: Staleness (mock-github)	4	execSync interception, comment count comparison, call log
5: loadCrossRefs validation	5	absent file, valid parse, invalid links, malformed JSON
Total	23	All passing

Test Plan

• vitest run test/validate-and-load.test.js — 23/23 tests pass
• No real .mgw/ directories created outside tmp dirs
• process.cwd() restored in afterEach for all describe blocks
• migrateProjectState() called twice — state unchanged on second run
• mock-github.cjs intercepts child_process.execSync correctly
• Group 4 tests degrade gracefully when mock-github.cjs absent (conditional require)
• All fixture files present in test/fixtures/github/ for mock activation

GSD Progress

Stage	Status
Triage	✓
Planning	✓
Execution	✓ (23 tests created, all passing)
Verification	✓
PR Creation	✓

[snipcodeit]

Testing Procedures

Prerequisites

• Node.js 18+
• vitest devDependency installed (npm install)
• The test/fixtures/github/ directory must exist (included in this PR)

Running Tests

# Run the new integration tests only
npx vitest run test/validate-and-load.test.js

# Run all vitest tests
npm run test

# Run in watch mode
npm run test:watch

# Run legacy node:test suite (not affected by this PR)
npm run test:node

Expected Output

 RUN  v2.x.x ...
 ✓ test/validate-and-load.test.js (23 tests)
 Test Files  1 passed (1)
       Tests  23 passed (23)

Manual Verification

To verify tmp directory isolation:

# Run tests and confirm no .mgw/ was created in repo root
npx vitest run test/validate-and-load.test.js
ls .mgw/  # should still only contain existing state files, none from tests

What Each Group Tests

1. Group 1 (T1.1–T1.5): Run and verify fresh .mgw/ init works in isolated tmp dir
2. Group 2 (T2.1–T2.5): Call migrateProjectState() twice — state should be identical both times
3. Group 3 (T3.1–T3.4): Verify loadActiveIssue() prefix pattern matching
4. Group 4 (T4.1–T4.4): Verify mock-github.cjs intercepts child_process.execSync
5. Group 5 (T5.1–T5.5): Verify loadCrossRefs() handles edge cases with warnings