Create lib/mock-github.cjs with GitHub API interceptors for tests

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Description

Implement a mock GitHub API layer that intercepts gh CLI calls and octokit requests during test runs. Must cover: issue CRUD, milestone CRUD, label operations, PR creation, and GraphQL board mutations. Responses should be configurable per test scenario via fixture files in test/fixtures/github/.

Acceptance Criteria

• Create lib/mock-github.cjs with GitHub API interceptors for tests

Milestone Context

Milestone	v6 — Local Testing & Simulation Framework
Phase	Phase 47: Mock Infrastructure
GSD Route	plan-phase
Phase Position	Phase 1 of 4 in milestone

Milestone Goal

Offline test suite with mock GitHub API and fake GSD agents to validate MGW orchestration logic without requiring live GitHub tokens or active Claude API calls.

Phase Goal

Build the foundational mock layer — GitHub API interceptors and fake GSD agent stubs that respond predictably so test scenarios can run fully offline.

Dependencies

none

Scope Hints

• Estimated route: plan-phase
• Labels: testing,infrastructure

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MGW · milestone-start · 2026-03-06T08:53:36Z

Milestone Execution Starting

v6 — Local Testing & Simulation Framework

Issues	11
Route	plan-phase (all)
Mode	Autonomous

Execution order:

#	Issue	Phase
1	#247 Create lib/mock-github.cjs	Mock Infrastructure
2	#248 Create lib/mock-gsd-agent.cjs	Mock Infrastructure
3	#249 Add vitest test runner	Mock Infrastructure
4	#250 Integration tests for validate_and_load	State & Pipeline Tests
5	#251 Tests for checkpoint read/write	State & Pipeline Tests
6	#252 Pipeline stage transition tests	State & Pipeline Tests
7	#253 Scenario tests for mgw:project	Command Scenario Tests
8	#254 Scenario tests for mgw:milestone	Command Scenario Tests
9	#255 Scenario tests for mgw:sync	Command Scenario Tests
10	#256 GitHub Actions test workflow	CI Integration & Coverage
11	#257 Vitest coverage reporting	CI Integration & Coverage

Issues will be executed sequentially. Each issue receives a work-started comment when its pipeline begins.

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MGW · work-started · 2026-03-06T08:53:51Z
Milestone: v6 — Local Testing & Simulation Framework | Phase 47: Mock Infrastructure

Work Started

Issue	#247 — Create lib/mock-github.cjs with GitHub API interceptors for tests
Route	plan-phase
Phase	47 — Mock Infrastructure
Milestone	v6 — Local Testing & Simulation Framework
Position	1 of 11

Pipeline executing autonomously...

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MGW · pr-ready · 2026-03-06T09:06:32Z
Milestone: v6 — Local Testing & Simulation Framework | Phase 47: Mock Infrastructure

PR Ready

PR #258 — #258

Testing procedures posted on the PR. This issue will auto-close when the PR is merged.

Milestone Progress (1/11 complete)

#	Issue	Status	PR
247	Create lib/mock-github.cjs	✓ Done	#258
248	Create lib/mock-gsd-agent.cjs	○ Pending	—
249	Add vitest test runner	○ Pending	—
250	Integration tests: validate_and_load	○ Pending	—
251	Tests: checkpoint read/write	○ Pending	—
252	Pipeline stage transition tests	○ Pending	—
253	Scenario tests: mgw:project	○ Pending	—
254	Scenario tests: mgw:milestone	○ Pending	—
255	Scenario tests: mgw:sync	○ Pending	—
256	GitHub Actions test workflow	○ Pending	—
257	Vitest coverage reporting	○ Pending	—