test: write unit tests for lib/state.cjs (#133)

[snipcodeit]

Summary

• Adds test/state.test.cjs with 44 unit tests covering all 9 exported functions in lib/state.cjs — the most critical module in MGW (every command reads/writes project state through it)
• Uses node:test + node:assert/strict (zero new dependencies), consistent with the existing test suite pattern in test/mgw.test.cjs and test/github.test.cjs
• Isolation strategy: fs.mkdtempSync() real temp dirs + process.cwd() override per describe block; afterEach cleans .mgw/ between tests

Closes #133

Milestone Context

• Milestone: v3.5 — Foundation Hardening
• Phase: 32 — Test Coverage
• Issue: 2 of 9 in milestone

Changes

• test/state.test.cjs (new, 546 lines) — 44 tests across 7 describe blocks:
  • getMgwDir / getActiveDir / getCompletedDir (4 tests) — path derivation from cwd
  • loadProjectState / writeProjectState (7 tests) — null on missing/invalid, round-trip, overwrite, auto-mkdir
  • loadActiveIssue (6 tests) — prefix matching, superset-prefix guard, invalid JSON, string vs number arg
  • mergeProjectState (7 tests) — throws on no state, milestone append, phase_map merge (no overwrite), new and legacy active pointer fields, disk persistence
  • migrateProjectState (8 tests) — null on no state, adds missing fields, does not overwrite existing values, idempotency, disk persistence, empty milestones edge case
  • resolveActiveMilestoneIndex (10 tests) — null/undefined state, new schema precedence, dangling reference returns -1, legacy conversion, neither field set, empty/missing milestones array

Test Plan

# Run full test suite — all 92 tests should pass
npm test

# Verify state tests in isolation
node --test test/state.test.cjs

# Confirm github and mgw tests are unaffected
node --test test/github.test.cjs test/mgw.test.cjs