refactor(tests): consolidate Milestone Module — 10 files → 4 (#3754)

* refactor(tests): consolidate Milestone Module — 10 files → 4

Closes #3753

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(changeset): correct fragment format for 3753-consolidate-milestone-tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(changeset): add required frontmatter to 3753 changeset fragment

Adds `type: Fixed` and `pr: 3753` frontmatter so docs-lint passes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(tests): bug-2943 execFileSync timeout 5s→15s for Windows starvation

PR #3754's milestone consolidation reshuffles run-tests.cjs chunk
composition. On Windows/Node 22 under --test-concurrency=4, bug-2943
subprocesses now share concurrency slots with bug-2760-codex-install
subtests (8–15s each), starving past the 5000ms timeout. 15s covers
the observed 13.5s worst case with headroom.

Refs #3753

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

.changeset/3753-consolidate-milestone-tests.md

@@ -0,0 +1,19 @@
+---
+type: Fixed
+pr: 3753
+---
+<!-- docs-exempt: internal test refactor only — no user-facing surface changed -->
+
+## Summary
+
+Consolidates the Milestone Module test cluster from 10 files to 4, bringing the cluster within the lint-test-file-count allowlist ceiling introduced in PR #3738.
+
+- Rewrote `tests/milestone.test.cjs` (869 → 720 LOC) with compact shared helpers, absorbing `milestone-regex-global.test.cjs` and `bug-3043-milestone-complete-scope.test.cjs`
+- New `tests/milestone-archive.test.cjs` (505 LOC) absorbs `bug-2684`, `bug-2787`, `bug-3164`, `bug-3600` — milestone archive layout, fenced code block parsing, phase filter, version forwarding
+- Expanded `tests/milestone-summary.test.cjs` (471 LOC) to absorb `milestone-audit.test.cjs`
+- `sdk/src/milestone-runner.test.ts` unchanged (421 LOC)
+- All 99 CJS assertions and 11 SDK assertions preserved (no regression)
+- Added Milestone Module glossary entry to `CONTEXT.md`
+- Allowlist update deferred to rebase after PR #3738 merges; pre-rebase, `milestone` entry would change from 10 → 4
+
+Closes #3753