test: write unit tests for lib/github.cjs (#134)

[snipcodeit]

Summary

• Adds test/github.test.cjs with 46 test cases covering all 10 exported functions in lib/github.cjs
• Uses the module cache invalidation mock pattern (delete require.cache + mock.method(childProcess, 'execSync', ...)) so no real gh CLI calls are made during test runs
• Every function has at least one error-path test confirming that execSync throws propagate correctly

Closes #134

Milestone Context

• Milestone: v3.5 — Foundation Hardening
• Phase: 32 — Test Coverage
• Issue: 1 of 9 in milestone

Changes

• test/github.test.cjs (new, 684 lines) — full unit test suite for lib/github.cjs
  • getRepo — 2 tests: correct command construction, error propagation
  • getIssue — 4 tests: parsed return value, command shape (number + fields), string arg, error path
  • listIssues — 8 tests: no-filter base case, each filter flag (--label, --milestone, --assignee, --state), assignee: "all" exclusion, error path
  • getMilestone — 3 tests: parsed return value, two-call sequence (getRepo then gh api), error path
  • getRateLimit — 4 tests: field values, command, no extra keys, error path
  • closeMilestone — 3 tests: parsed return value, PATCH command with state=closed, error path
  • createRelease — 7 tests: raw string return, base command, --notes, --draft, --prerelease, false opts omission, error path
  • createProject — 3 tests: {number, url} extraction, command shape, error path
  • addItemToProject — 3 tests: raw string return, command shape, error path
  • postMilestoneStartAnnouncement — 9 tests: Discussion success, no-Announcements-category fallback, GraphQL-throws fallback, both-fail {posted:false}, no-repo no-op, boardUrl in body, "not configured" when absent, issue rows in body, em-dash for unassigned

Test Plan

• Checkout branch, run npm test
• Confirm output shows tests 50 / pass 50 / fail 0
• Confirm no network or gh CLI invocations occur (all mocked)

[snipcodeit]

We need to add a deployment ci cd run this to ensure nothing breaks.