test(project): add scenario tests for all six mgw:project STATE_CLASS paths

[snipcodeit]

Summary

• Extracts detectProjectState() into lib/state.cjs — encapsulates the five-signal classification logic from workflows/detect-state.md as a pure JavaScript function (no shell calls)
• Writes test/project-state-detection.test.js with 19 vitest scenario tests — one describe block per STATE_CLASS (Fresh, GSD-Only, GSD-Mid-Exec, Aligned, Diverged, Extend) plus return-shape contract tests
• Includes full vitest infrastructure from the prior PR chain (Create lib/mock-github.cjs with GitHub API interceptors for tests #247–Write pipeline stage transition tests for mgw:run via mock agents #252) since those PRs haven't merged to main yet

Closes #253

Milestone Context

• Milestone: v6 — Local Testing & Simulation Framework
• Phase: Phase 49: Command Scenario Tests (Phase 3 of 4 in milestone)
• Dependencies: Blocked by Write pipeline stage transition tests for mgw:run via mock agents #252 (pipeline transition tests) → PR test(pipeline): add pipeline stage transition tests for mgw:run #263

Changes

lib/state.cjs — add detectProjectState()

• New exported function with signature detectProjectState({ repoRoot?, githubMilestoneCount? })
• Reads P/R/S/M signals from filesystem; G signal passed as parameter (caller owns gh api)
• Classification: Extend → Aligned/Diverged → GSD-Mid-Exec/GSD-Only/Fresh
• Consistency check uses count proximity (|local - G| <= 1) instead of live name-overlap API call

test/project-state-detection.test.js (new)

• 19 tests across 7 describe blocks
• Isolation: fs.mkdtempSync() tmp dirs, file seeding helpers, afterAll cleanup
• Fixture-based project.json loading via seedProjectJson()

test/fixtures/project-state/ (new)

• aligned.json — 2 milestones, current_milestone=1 (not all done)
• diverged.json — 5 milestones, current_milestone=1 (count diff > 1 vs G=2)
• extend.json — 2 milestones, current_milestone=3 (all done)

Vitest infrastructure (from PR chain #247–#252, included since not yet on main)

• vitest.config.js, lib/mock-github.cjs, lib/mock-gsd-agent.cjs
• test/loadFixture.js, test/setup.js
• test/checkpoint.test.js, test/pipeline-transitions.test.js
• test/fixtures/github/ (16 fixtures), test/fixtures/agents/ (5 fixtures)
• Updated package.json: test script → vitest run, added vitest ^2.1.9 devDep

Test Plan

• npx vitest run test/project-state-detection.test.js → 19 tests pass
• npx vitest run → 83 total tests pass (19 + 28 pipeline-transitions + 36 checkpoint)
• Fresh: empty tmpDir, all signals false → stateClass === 'Fresh'
• GSD-Only: only .planning/PROJECT.md seeded, G=0 → stateClass === 'GSD-Only'
• GSD-Mid-Exec: PROJECT.md + ROADMAP.md + STATE.md, G=0 → stateClass === 'GSD-Mid-Exec'
• Aligned: 2-milestone project.json, G=2 → stateClass === 'Aligned'
• Diverged: 5-milestone project.json, G=2 (diff=3 > 1) → stateClass === 'Diverged'
• Extend: project.json current_milestone=3 > milestones.length=2, G=2 → stateClass === 'Extend'
• detectProjectState exported from lib/state.cjs module.exports

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